if someone gave money as a maaser check and the check is cashed by a money changer in Eretz Yisroel – the institution got the money – but the money changer lost the check, so the money is still sitting in the donor’s account, is he patur from giving maaser? is he chayav in giving the money to the money changer? If he is still chayav in maaser, is he obligated in giving the money to the money changer or can he give the maaser somewhere else?
No, he is not patur from Maaser, as he never gave the money.
Yes, he must give the money changer the money, as he was giving money on your behalf and at your request. Even though he can no longer prove it you are obligated to fulfill your commitment.