The payment was applied to the prior bill, hence the yellow envelope which means was billed. If you are saying that the current balance is incorrect, then there's an issue but the payment was already applied.

Right, that's what I said. The payment was applied to a prior bill, and reflected in the next bill, but not this current one (the balance) and also, on the current bill, it shows the invoice that this payment was applied to as a past due amount...does that make sense?

Sorry, doesn't make sense to me. Maybe you can give us some dummy numbers or maybe someone else will understand.

Invoice 1 - Total of Slips: 2500.00

Invoice 2 - Total of Slips: 1000.00
Previous Balance: 2500.00
Total: 3500.00
Payment Applied: - 2500.00
Balance Due: 1000.00

Invoice 3 - Total of Slips: 1000.00
Previous Balance: 1000.00
Balance Due: 2000.00

Invoice 4 - Total of Slips: 1000.00
Previous Balance: 4500.00**
Balance Due: 5500.00

**As you can see, the most recent invoice has a balance that includes the amount of the first invoice, that was paid, and accounted for in the following invoices leading up to the current (Invoice 4).

Also, at the bottom of the current invoice, where it shows outstanding amounts, under "90 days" it lists the amount of Invoice 1, i.e. 2500.00

Now I go it. My guess is that you have some corruption. There is a process through database verification to rebuild the AR balance. Absolute rules
(1) MAKE A BACKUP BEFORE YOU DO THIS
(2) ONLY REBUILD THE AR BALANCE FOR THE ONE CLIENT
After you finish test. If this worked then you can move forward. If it did not work then restore the backup.