What should I do regarding a debt of 40 [currency unit] that I owed to a Christian who died without heirs?

The fundamental principle concerning money is to return it to its rightful owners, whether they are Muslims or non-Muslims. If the owner's death is confirmed, it should be given to their heirs. If none of the heirs can be identified, it should be spent on charitable causes with the intention of absolving oneself of the obligation towards others' rights.