Based on still having a Customer credits account, you are thousands of versions behind. Update your software. Customer credits was integrated with Accounts receivable in July 2016. See [16.7.73] Categorization against customer and supplier accounts more intuitive.
Spend Money transactions were also long ago replaced with payments.
When you update, all your data will be converted to current standards. The same capabilities, and many more, are available.
What used to show as a liability in Customer credits will now be a negative contribution to Accounts receivable. If a customer wants a refund, you don’t enter that with a credit note. A prior credit note is probably why they have a credit balance. You just enter a payment, allocated to Accounts receivable (after the update). Read the Guide: Pay a refund | Manager.