I think giving versus taking materially changes the ethics of the scenario. If someone gives you something as payment, no strings attached and no misrepresenta

I think giving versus taking materially changes the ethics of the scenario. If someone gives you something as payment, no strings attached and no misrepresentation, then it's a fair play. They gave you a watch as payment, they didn't ask you to sell it and pay back the difference. The debt is settled even if the watch isn't worth enough, because it was the payment, whatever its value ends up being. If you took the watch to repay a debt, then yeah, it's unethical not to pay back the difference. Thinking about it in the converse helps: if you took the watch and it wasn't worth enough to pay back the debt, you would probably insist they're still on the hook for the rest. Cuts both ways.