Your depositor asks you for reimbursement for the check because, with the forged endorsement, it was not properly payable. You in turn enter a claim with the depositary bank, based on that bank's transfer warranties under the UCC.
Then you wait for the depositary bank to figure out that it actually owes you the money. If the check is relatively small, be prepared to be ignored (some banks have taken this approach, even though it means they are reneging on the warranty). The depositary bank may bet that you won't spend on a lawsuit for a small potatoes check. Be prepared to consider calling in an attorney if the dollars are worth it, particularly if you're headed over state lines with your claim.
In the meantime, you may decide to reimburse your customer before getting the funds from the depositary bank.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8