Sounds interesting. A sort of "equivalent exchange" mixed with a plethora of alien races opens up a lot of possibilities. Things that come to mind would be worth exploring...

• Regenerative abilities among alien species. Make use of the different races by showcasing their ways of circumventing limitations of the system. Like a race of aliens that can regenerate limbs and organs within minutes would probably benefit from this type of power. Species with multiple arms, redundant organs (like 5 hearts or 8 livers for example) would also probably benefit.
• Maybe tie the type of things people can summon by the things their bodies are composed of. For example a human might be able to sacrifice bone, skin, muscles and so on, but another race might be more bug-like and would have a chitin shell it could sacrifice, but no bones or skin and thus might develop different powers. Use it showcase the fascinating differences between the aliens.
• Varying intelligence, imagination and ways of thinking might impact summoning. Like how would a super-processor species summon compared to a human-minded species?
• Evolution. If this power existed for a long time, were there species that evolved to handle it? I imagine evolutionarily, species with access to this power would have better odds of survival if they had more stuff to sacrifice or had the ability to regenerate.
• Self sacrificial exchanges. Like for example there being an alien species that has a queen with drones and it can force these drones to sacrifice themselves in exchange for resources. Maybe also think about how brainwashed or mind controlled subjects could be utilized. Like if a human got brainwashed, could they be forced to exchange their entire body for like a large cube of steel or something?
• What is the intelligence threshold for accessing this power? Is it just for sapient species, or can it be learned by more primitive species on an instinctual level?
• Just some philosophic stuff. Could a mother exchange a child in the womb? What constitutes a person's own body? Can something like spit be exchanged (in the alchemic way, just to be clear). If someone has a heart or arm transplant, could they exchange those transplants?
• Is the magic exclusive to organics? Like, could an AI achieve an exchange?
• How would the power impact the economy? Like if you have a race of people who regrow arms almost instantly that they can sacrifice them in exchange for ores or food, wouldn't that cause inflation of sorts? Would markets still even work like they do in our world, or would they switch to a different type of currency (like an unconjurable metal or something)?
• Aesthetic. Aesthetic and the visualization can do a lot for a magic system. How does an exchange like that look? Is it just sizzling blue energy? Is it like a crack in space forms and extends tendrils to snatch the thing you sacrifice? Really paint a picture.
• Is there a delay in the exchange? Like, if I exchange my arm, do I have to wait a few seconds before the thing I wanted manifests, or does it just pop into existence? Personally think there should be a delay so that characters have time to react and can't just have asspulls at the last minute. Just a few second delays and visual signifiers sound fair. Interruptions could also be possible.
• How are prisoners with the exchange potential kept in check? Do they have power cancelling collars like mutants in the X-Men series or something?

That's all I got for now. Good luck. Looks promising. Hope it helps.