Here’s my core constraint:
When the next person receives BTC (either through trade or withdrawal), it must be the exact same BTC originally deposited by John — not from James or a general pool.
So when Harry withdraws, I want to ensure the withdrawal is signed using John’s wallet key, since that UTXO technically still resides in John’s wallet (now owned by Harry).
It's it too complicated?
I sold BTC to you and now you are waiting my approval to sell/withdraw the BTC. The relation between me and you should be completed once I release the BTC and it goes to your account (internal balance). With that balance whatever you do, why would you need my involvement.
You will give the address to your users but they private key is held by you?