What must a person do who unknowingly fell into currency exchange usury with a woman, and the amount is still in his possession, and a month has passed since the transaction?

For the exchange to be valid, possession must be taken in the contracting session. If this does not occur, the sale is void, and both exchanged currencies must be returned. The Sunnah of the Prophet has indicated the obligation of returning items in usurious and void sales. If it is not possible to return both currencies after some time, the questioner is not obligated to donate the profit, because he did not intentionally commit the transgression, and because this profit is not the woman's right, while noting that donating some of the profit is an act of goodness and favor.