Same way you don't get back the exact serial-numbered dollars you loan (deposit) to a bank. It's not the _identity_ of the bitcoin which matters, it's the value.
If you want a lockbox, get a lockbox. If you want a deposit account, you're storing and retrieving fully fungible and interchangeable entities.
if i borrowed 1 bitcoin from you, and then i default, then how are you going to pay back that bitcoin to whoever deposited it?