Is it permissible to retract the waiving of a debt owed by a deceased person after verbally declaring it, with the intention of reclaiming it in the future if the heirs' financial situation improves, and is he entitled to take it if the heirs repay it of their own accord?

If what happened was merely a fleeting thought and a self-dialogue that you did not utter, then there is no blame upon you for demanding the debt from the heirs, for the Prophet, peace and blessings be upon him, said: "Indeed, Allah has pardoned my Ummah for whatever occurs to their inner selves, as long as they do not act upon it or speak of it." However, if you verbally pronounced the discharge, then it is binding, and the debt is waived from the deceased, and you are not permitted to demand it from his heirs, because what has been waived does not return.