Answers to this question link several other Q&As on the theme that if someone accidentally sends you a pile of money, you need to give it back. Fair enough.
If someone sends me $100k accidentally, and a month later asks for it back, how much do I owe them? More specifically, if I hold it in an interest-bearing account, but return it promptly on demand, who keeps the interest?
(Assume for simplification that I had no way to notify the payer in a more timely way, but return the original amount promptly on demand).
Asking about US law, but other jurisdictions might be interesting too.