To reverse a posted check in Adagio Payables, you need to find the check on the Check Reconciliation screen - this can be found from the Payments menu. When the check is highlighted in the grid, click the Reconcile button and choose Reverse. Not only will the check amount be reversed, but any invoices that had been paid by the check will be unpaid.

Note that if the payment no longer exists in either Current or Historical transactions for the vendor, then the Reverse function will not be available.