As a Service Provider, when you refund invoices, it’s possible your Available Balance will drop below $0.
As a safety measure to stop fraud, refunds are always deducted from the Available Balance.
The pending balances from refunded invoices will move from pending to available as normal, which will cancel out the negative balance. You have the option of paying the negative balance off with your credit card, but there is no need! Instead, wait until your pending balances move over to Available and cancel it out.