Actually, while I still hold to my below reply, I'd like to add that I find The Prisoner's Dilemma to be the foundational question of morality. The purpose of morality is to persuade/coerce everyone to always cooperate in Prisoner's Dilemma situations (simplification, but that's the essence). If you find money in a shared living space, cooperation = trying to return it, defecting = keeping it. Unless there's extreme justifying circumstances (which you shouldn't trust anyway, we are all running on corrupted hardware, better to let a disinterested 3rd party decide what counts) you should cooperate.

Not being a dick. Some moral dilemmas are actually tough. Unless you're starving, or have some other immediate and extremely-high-utility use for the money you didn't mention, this isn't one of them.