I note a number of 2017 invoices indicated they had been ‘overpaid’. The ‘over paid’ amounts were actually allocated to the 2018 invoices. In other words, when the 2017 invoices were paid in full, the additional payments were allocated to the next invoice (2018) however, for some reason, these are appearing against 2017 invoices.
Are you referring to sales or purchase invoices? The term payment in Manager means an outgoing movement of money. If you are referring to sales invoices, you record incoming money as a receipt.
Whichever you mean, can you post screen shots showing the monetary transaction (the Edit screen, not the View screen) and the list of invoices showing the status of the invoices in question. If there was a “chain” effect of rolling allocations, you may need to show more than one transaction.
Finally, please describe exactly how “the additional payments were allocated to the next invoice….” In other words, are you saying Manager did this automatically, or that you did it without the intended result?