If they destroy your check and don't keep an image, they lose the authority to draw money from your account without your permission. You have the right to dispute the transaction. I suspect that your use of their system contains an agreement to their terms, which state that they have the right to treat it as an electronic transaction and destroy the check. They should then provide you with an electronic record of the transaction to make it enforceable. Our banking system works because you have the right to dispute any withdrawal from your account and the person making the withdrawal has the burden of proving the transaction.