Manager 18.1.6

[sharpdrivetek]

i suppose you had not updated Manager for sometime now.

there had been changes to how credit is allocated to invoices.
you can read about the same in below topic.

[17.9.28] Improvements to automatic credit allocations

Implementation of automatic credit allocation in Manager was a concept which was always causing issues among users. The latest version is attempting to solve these issues. Eliminating automatic overpayment entries In theory, automatic allocation of overpayments keeps accounts receivable and accounts payable tidy even if bookkeeping errors occur because overpayments on one invoice are automatically re-allocated to different invoice. This means invoices are either paid or unpaid - never overpaid…

@jimmy

Now I have an invoice from November 2016 showing overpaid by $100.77, then on a different invoice from November 2016 too, the same amount of $100.77 is overdue.

you must have received a payment which was $100.77 more than the invoice value of the invoice selected. edit the receive money transaction you had made and delete the invoice number under the invoice column. this should fix the issue.