Am I religiously obligated to repay the debt in Qatari Riyals or US Dollars, or in Yemeni Riyals, considering that my son sent me a portion of the debt owed to him by the debtor, and the debtor is demanding that I repay what I borrowed from him years ago?

It is obligatory to pay with the Yemeni Dinar because it was the currency requested initially, the one the borrower received, and the one that burdened his financial liability, not the Qatari Riyal. The borrower did not request or receive the Qatari Riyal. An analogy for this is someone who requested and received a garment; he is only obligated to return the garment. This is supported by the hadith: "On the hand is what it has taken until it renders it back." If the Yemeni Dinar is available, it must be returned regardless of its value. If it is not available, then its value must be assessed on the day it was lost, based on what it equals in a stable currency.