All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If the money that you will take upon returning to your country in compensation for what you paid to this man is in the possession of a specific entity or particular person to whom this debtor has referred you in order to take your money, then there is no harm in this, as it apparently does not involve any forbidden act. The basic principle in transactions is permissibility, unless there is evidence indicating their prohibition. However, if the zakat money which they promised you was not in the possession of a certain person or particular body, then you are not entitled to ask people to give you from zakat money in compensation for the money that you gave to this man, as what you gave him is considered a loan. The claim of one's loan should be from the one who took the loan himself or someone else to whom he has given authorization.
In the case of debt assignment, the assignor should transfer the debtor to a specified third party (i.e. debt assignee). In your case, the debt assignee is not specified. Hence, the debt assignment is not valid and you have the right to ask the man who took your money to repay the money that he owes you.
Allaah Knows best.