How can one atone for the sin of taking a small sum of money from the purse of one's deceased wife's mother, and is it permissible to return the money by placing it in her clothes in the closet?

You must return the money you took to the clothes of the deceased in the closet, and alert her heirs, or one of them, to the presence of money with the clothes if they have not noticed. You are not obligated to inform them that you took it and returned it. Explain to them that the clothes are part of the estate and must be divided or sold and their price divided, unless they all waive their right to them, in which case it is commendable for them to donate them as charity. And whoever repents, Allah will accept his repentance.