We can run the Open Balance vendor report to see if there's an unapplied credit or overpayment, eas22487. 

In the Vendor Center, right-click on the vendor's name and select Open Balance: (Vendor's Name). If the unapplied credit is showing on the report and doesn't show when applying credit, there might be a data damage. Let's run the verify rebuild data utility to fix this. 

However, if it didn't show on the report, we can open the Bill Payment to delete and then recreate. The credit should now show on the Apply Credits window. 

Let me know if you need more help with applying vendor refunds or credit. I'm here to help you!