I think that scenario is pretty unlikely for large exchanges.
If someone gave me a copy of a private key, corresponding to some bitcoins, I wouldn't consider it payment, as they still have the private key as well (meaning they could still spend the bitcoins at any time).
If someone gave me a copy of a private key, corresponding to some bitcoins, I wouldn't consider it payment, as they still have the private key as well (meaning they could still spend the bitcoins at any time).
It's more like a "shared account" at that point.