Is it permissible to accept monthly financial assistance from someone who previously wanted to marry you, while concealing the matter from your current husband, knowing that these funds are not in exchange for anything?

You must cut ties with this foreign man to avoid corruption. Speaking with him is a forbidden, satanic step, unless there is a need and only to the extent of that need. As for accepting money, there is no impediment to it if it is with good intentions and not for achieving corrupt purposes, like a gift intended to entice a woman into committing forbidden acts.