Is a woman sinful for returning a debt to another woman with money she found in Mecca?

It is not permissible for the lender to take back their debt if they know that the borrower repaid it from unlawful gains, because dealing with unlawful gains itself is forbidden. However, if the source of the money is unknown, or if it is mixed with lawful and unlawful money, then it is permissible to take it.

As for the woman, if she has announced the found item (Luqata) of the Haram for a year, then it is permissible for her to take it and pay off her debt from it. Otherwise, it is not permissible for her to own it or pay off the debt from it, and what she takes would be unlawful. It is also not permissible for the lender to take it if she knows this. However, if she only has doubts, there is no blame upon her for taking it.