For the specific instance of the one you already bought, not that I'm aware of, since that's a unique generation of stats/traits. You can't "return" them, just eliminate... you bastard.
I assume the situation is, you bought Amelia, decided would rather have someone else for now, want to be able to buy her again. That can be arranged in a quick and dirty way, though it might break things along the way. Use at your own risk, which probably includes potential save corruption. I only tested whether or not the slave is available for purchase again. This only works for slaves you do purchase, any other unique slave would need resetting of all their quests variables, and I never messed with my games this much (I use my own advice and limit cheating to setting up "thematic" characters and occasional quick-starting).
One thing to mention is that you should absolutely NOT use this method for generating multiple copies of unique slaves. This is damn-well-guaranteed to break things up with normal gameplay, since the game tracks multiple things including state of their personal quests and you'll have two different "characters" trying to store their individual values in same variables.
Anyway, to reset ownership of a unique slave, you need to look up the ownership variable for them. Meaning if it says in the copypasta below $ownlov, the Sugarcube invocation to "reset" Lovisa availability at the market is "SugarCube.State.active.variables.ownlov = 0" (without quotes). Remove their presence from your household using Talk->End Slave Contract (this might also affect Happiness/Affection of other slaves in your household, but it's the easiest way). Then reset the appropriate ownership variable, and that character should be available for purchase again.
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Above contain slaves you should NOT touch (Juno likely, Michelle, Aria, whoever else is not directly purchased from one of the slave markets without involvement of a quest).
It's not that it's impossible to "reset" non-purchaseable slaves, or Aria (it is), but you'd need to know every single game variable that needs to be changed and that's a lot of digging through the code. I encourage you to experiment (on a separate save!), because it's a good way to learn Sugarcube and JavaScript. Which makes it a doorway into learning how to make your own stuff within same environment. Or just to know more things you never know when might come in useful.
Sadly, it's just not something I can throw my time at to help with in broader scope, sorry.
