The currency exchange is trying to take the lazy approach and go after the issuer of the check. That tactic is correct when the reason the check was returned is that payment was stopped. But this is a different scenario.
The check was issued to the payee, who received value for the check when it was mobile deposited. The issuer of the check paid for the check when the mobile-captured image of the check was paid. The payment is complete, and the check issuer is no longer liable on the check -- to anyone.
The payee is the only person the currency exchange has a claim against, based on his indorsement and negotiation of the check to the currency exchange. Since the payee is the person who was paid twice, the result is fair.