New Spell/Arcanum: Chimeric Exchange

Years ago I came across this thread where people brain-stormed magic systems. And one of them stuck in my brain for years now, accumulating more details and thoughts. So I'm making it into a specific spell with a place in the world. The entry was this one:
"43. Body Parts
Inspired by a post in the "Why do wizards wear robes?" thread ...
People can't do magic. In fact nobody can really "do" magic, but creatures from other realms have properties that could be described as magical - and those properties are a part of their body. 
An air elemental flies on wings of lightning more manueverable than a hummingbird and faster than an arrow. A salamander has arms literally made of fire, that can stretch to fill an entire courtyard. And if you had those parts, you could do it too.

Not by stealing the parts and stitching them on (although that would be a possible magic system itself); that's not how the ritual works. But you can trade them, and many creatures are willing to make that trade. For them, the properties humans have are magical - things like being able to stand still when the wind blows or touch things without burning them. 

Sometimes, the parts you get in return are a straight-up substitute, but often they're insubstantial, or entirely different. So senior wizards tend to have a lot of prosthetics (hence the voluminous robes to cover up how strange they start looking). A master mage might have false legs (moves around by flying), one arm made of shadow, no tongue (communicates via writing and an assistant), and lives off heat from fires instead of eating food. Making the right choice is crucial - woe to the mage who trades a unique body part for something worthless. Assistants are always useful - while you can't trade their parts for more of your own, you can convince them to make the trades you need and use the resulting abilities on your behalf."

Chimeric Exchange

Level whatever spell

An object similar to a Chinese finger trap, but metal and not woven. (if it's an object)

As a spell the caster selects two parties. If the these two parties come to an agreement, than they can trade body parts between themselves with the same ease that item are exchanged. The deal is finalized with a hand shake (or equivalent custom that both parties except). 

As an item, it works the same except negotiations don't need to be identified before hand and the deal is sealed by both parties inserting an appendage into one of the wholes.

• For it to work, both parties need to agree to the exchange.
• The spell doesn't distinguish between willing agreement and coercion.
• Knowing the specifics of what your giving up and gaining isn't required.
• The exchange can be one-sided. For example, the spell allows you to give someone an extra eye without taking something from them, if both of you agree.
• You can trade goods and services that aren't body parts. This is equivalent to a one-sided exchange as far as the spell is concerned. In the above example, you can demand compensation of 10 gold for giving away your eye.
• As a spell, the caster doesn't need to be one of the two parties but they can be.
• Once an agreement is reached, the tendrils of etherealness appear and remove the agreed upon body parts and attach them to their new owner. While this is happening both parties are effectively petrified.
• If interrupted, both parties are unpetrified and die slowly from blood loss or immediately depending on which organs are exposed. If you were exchanging an eye, you'd probably survive with bandages and will eventually need a patch or glass eye to cover it up. If you were switching hearts, than you're screwed.
• The organs can be placed either where the agreement specifys or what makes most sense if not specified.
• After the spell does it's magic, a party may die if it suddenly doesn't have a vital organ. If you gave someone lungs without receiving some in return, you're going to start suffocating from not having anything to breath with.
• There are not scars or stitches left from this spell.
• Organs that are gained through this spell kind of become part of the person, kind of don't.
• It takes anywhere from a few weeks to a life time to get used to new body parts, based on myriad of factors.
• Younger people generally get used to it quicker for example.
• Beings changed by this spell do not breed true. Procreation results in the offspring using genetic code of whoever the organs originally belonged too. If I sell my testicles or ovaries, it means that anyone born with them are biologically my children.

You can ignore all the bullet-points if want to as the spell description is enought to go off of. The bullet-points are additional ideas that have accumulated over time as the ideas has bounced around in my head. 

World-Building Implications and Possiblities

• Wizards hide under large cloaks and big hats to hide their weird transmogrified bodies.
• Wizards that don't use this spell still do it to maintain the mystery.
• There are physically imposing opponents that none the less have great weaknesses. 
• Due to trading his liver for the strength of ten men, this person has to be vary careful with his diet.
• It's common knowledge that deals with wizards is full of peril.
• A variety of hybrid monsters have some interesting story behind them.
•  The Manticore is one of those bastards who demands you give him your best physical features on threat of eating you, than proceeds to eat you afterwards anyway.
• People want him dead for the obvious reasons, but other chimeric mages really want him dead for besmirching their image.
• Manticore is also an incredibly petty bastard.
• When the horse nomads came from the steppe to attack our lands, we we're amazed by how the horse and rider moved as one. To counteract whatever sorcery they use to achieve this, we have merged together our strongest steeds and most greatest warriors to produce the centaurs. It is a great sacrifice and those who under go it are noble men indeed.
• Nobody has the heart to tell the centaur's nation that the magic used by steppe nomads is known as "riding on horseback from the age of three".
• The Sphinx is also a lion-bodied human who eats people. However unlike fucking Manticore, she doesn't eat people from her country of not-Egypt and targets not-Greece instead. 
• There is conversely a lion somewhere who is enjoying having opposable thumbs while Sphinx out there eating people.
• Harpies are a group of birds that wanted human faces.
• You may wonder who would enought beings to agree that the Chimera should exist like it does. Don't, really don't want to know why this came about.
• Hybrid creatures that you want to be (un)natural without this spell could still exist, but I would make them "less hybridy". For example merfolk could be made either more mammalian, by making their tail more dolphin-like, or more fish-like like all those creepy mermaid illustrations.
• Different cultures can have different opinions on this magic.
• In one place this magic is despised and reviled. Mages who use it are assumed to steal the organs that they gain from it.
• In another culture the the practice is allowed and has become a part of fashion. Blue eyes are in season right now. Generally only the wealthy have the opportunity to mess around like this.
• In some places they choose a champion who receives the best organs from willing donors to represent the entire community in a sport or a war.
• The donors generally swap out organs with the champion and don't die from this.
• Some kings, priests and heroes have have lived beyond mortal years by swapping out bits of themselves Ship of Theseus style.
• In places the spell is known the slave trade includes the consideration of person's organs as having separate worth.
• In places that the spell is not demonized there is variation in whether or not it's ok to change you body shape to something nonhuman.
• The desperate will give away parts of themselves to survive or pay a dept.
• Artists would give up their painting hands to pay for things, having to rebuild the muscle memory from scratch.
• A teenager sell their face for an ugly one and a cold bag of coins.
• Spies could be anywhere and they can hide under anyone's face.
• They often don't but the possibility is still there.
• Grandpa lost his leg in a battle, so he could support the family he secretly went to a witch with a goat. She replaced the stump with the goat's hind leg and accepted the rest of the animal as payment for services rendered. Grandpa is deeply ashamed of having a goat leg and hides it from everybody. While he still walks with a hobble, at least he's able to support his family.
• The witch Matilda has cat ears instead of human ears. She replaced her ears with those of her familiar's back when she was an novice and has worn them ever sense. Even thought human ears make Matilda's familiar look kind of silly, he's accepted this because it makes Matilda so happy to have them. 
• That won't stop him from complaining about it thought, Matilda is wanted for heresy in several countries and they could use a lower profile.
• The dragon Zorsass has is the greatest opera singer in the city. They achieved this by trading away most of their body for a smaller and sleeker human form, the best voice in the world, large powerful lungs, a fully staffed mansion and enought bribes to become a member in the most exclusive of clubs.
• If encounter someone with dragon organs in the city, they probably once belonged to Zorsass.
• Other dragons are baffled and disturbed by Zorsass's life choices. Best to avoid the topic.
• As a dragon Zorsass has not spent a penny of his or her horde, they say that it's hidden in the depths of the dungeon.
• Zorsass may look fairly human, but underneath they are still a dragon, and could easily tear you in half with elegant grace. Will also lounge around like a dragon when guests aren't over.
• PCs can sell the random mutations they receive from chaos magic and potions as treasure.
• Clever PCs can improve their stats with negotiations.
• Losing your humanity in the pursuit of power has never been so literal.
• On the flip side of the last point, beware the Cat with Hands.

You could have a story based around solving someone's gender dysphoria by going on a quest to find people to trade away the body parts with. However I'm not sure how to handle that respectfully. Depending on how you handle it, this spell could lead to some dark places or mythical tales or gonzo shenanigans with puns.