When you get the check copy and verify beyond a doubt that it was not the intended biller who cashed the check, re-open the case with Fiserv.  Explain that the check was wrongfully deposited, explain your verification processes, and ask for Fiserv's assistance with a refund to the member.  They will contact the Federal Reserve and work to get the money back. This can only be done with a Corporate Check. If it was a Member Draft, then the credit union would need to assist the member in reclaiming their funds.