Good day mvroadrockin,
It’s possible that you’ve manually recorded a payment for your client's invoice.
To make sure, we can start by checking your client’s invoice. This way, we can verify if there's a payment linked to invoice. Once you've found out, you’ll have to delete the payment. Here’s how:
Now that you're done doing the steps above, you can advice your client to refresh the invoice and click the View and Pay button in the email.
If you have follow-up questions, please post them right away to his thread. I'm always here to help you!