Write update in more efficient way with joins
You can use an INNER JOIN on CurrencyID to join table1 back on itself but filter the table being updated to records where accountID = @recipientID and the joined table is filtered where accountID = @payerID

In this fashion, only record meeting the WHERE clause for both tables that also matches CurrencyID values will be included in the UPDATE.

UPDATE t1
SET t1.balance = t1.balance + @amount
FROM table1 t1
INNER JOIN table1 t2 ON t1.CurrencyID = t2.CurrencyID
WHERE t1.accountID = @recipientID
    AND t2.accountID = @payerID