Is it permissible for the lender to reclaim the loan in Saudi Riyals, despite being compelled to convert it to Yemeni Riyals upon sending it, considering himself an agent of the borrower in currency conversion?

The principle in fulfilling debts is by likeness, not by value. As long as the transaction for the loan was in Saudi currency (2000 Saudi Riyals), that amount remains fixed in the debtor's responsibility. The conversion to Yemeni currency would be on behalf of the borrower if there was customary or verbal permission. However, if both parties agree at the time of repayment to pay in another currency, this is permissible.