Legally - you've already realised they've paid you extra so you couldn't spend it and say you'd changed your circumstances in reliance on it ie you don't have a defence in law if they ask for it back and you've spent it. Ethically, you know it's not yours (hypothetically or otherwise!). Practically, I'm sure most people would just keep schtum, but be prepared not to have a leg to stand on if/when they cotton on and ask for it back! So if you want to throw it in an interest-bearing account so you know it's there as and when they ask for it (while you get to keep the interest, they can't ask for that back), go right ahead. If you fancy spending it ... hmm, you know what the answer is!