What should a person do with money acquired from reciting the Quran for the deceased, which was given to him without a predetermined price?

If you took money in exchange for reciting the Quran for the deceased—which is an innovated and forbidden act—then you must return it to its owners if you know who they are. Otherwise, you must donate it to the poor, the needy, and for the general welfare of Muslims, with the intention of giving charity on their behalf, because it is ill-gotten wealth that must be disposed of.