Is it considered usury (riba) if a debt is repaid in a currency different from the lending currency, and is it permissible to take the remaining surplus and give it away as charity?

There is no harm for the borrower in returning the debt with a currency other than the one he borrowed, provided that there was no prior agreement at the time of the loan for this, and provided that he returns it at the exchange rate on the day of repayment, and that they part ways with nothing owed between them. Transferring or depositing into an account is considered constructive possession. If an agreement for currency exchange (saraf) is made before the amount is paid, then immediate possession (taqabud) must take place in the same session. If you send the amount with an agent and agree on currency exchange upon the agent's arrival and receipt of the amount, then this is permissible. The increase, meaning it is more than the lender's right in the loan currency according to the exchange rate on that day, is not permissible for him to take. If the increase is a trust with you and you instruct him to donate it as charity, there is no harm in that, as long as you did not pay it to him with the intention of transferring ownership to him as repayment for the debt.