What’s really stupid is that they could have turned the money and made a profit on something and then return the money, but instead, they spent it. Very, very foolish.
If someone gives you money by mistake you hold it in “constructive trust”. Receiving the money into the account is not theft, but withdrawing it and spending it is.
Because they are constructively trustees of the rightful owner, if they they turned a profit on the money while they have it, they are investing it for the owner, and the profit goes to the owner.
These are all concepts that were pretty much worked out well over 100 years ago.
They probably figured if they spent it all and gave a lot of it away, nobody could prove they ever had it.
Your solution is a lot smarter.
Good plot for a TV crime show.
But I bet they’d make you pay whatever interest the money earned as well.