Credit on account

Ive had a quick search through and was unable to find this topic. Hoping someone can help!
I have an accounts receivable that overpaid two months in a row.
Manager has automatically used the second receipt amount for an automatic payment for an invoice, leaving a balance for the next one, however the first receipt that was overpaid has not been auto paid to any invoice. It still sits as an overpayment.
How can I pay that off the next invoice if it does not do it automatically?

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