returned checks

Does anyone know whether we can redeposit a returned check from a member donation?

You should not try and redeposit a returned check - it should be returned to the member if you have the check. The process is to delete the original donation that was made with the returned check and then the member has the option to make the donation again. If the member does make the donation again, it should be with a new check and it is processed by the ward like any other donation.