ArgumentException: Invoice # '***' for vendor 'VendorName' is division ** is already posted, please use invoice adjustment to manually enter this invoice.

Common Cause:

The invoice sync error means the invoice is flagged as a potential duplicate. This means that the invoice number and vendor coding already exists in Sage 100 and is posted.

Error in tool

Resolution

You should confirm that the invoice is in fact a duplicate invoice. If you find the invoice in Sage 100 is the same invoice, you can simply Mark the Invoice as Exported in SageAPA. Since the data is already in Sage 100, you don't need to export it again.

It's possible that the invoice is actually a different invoice, this sometimes happens with vendors that are poor at using unique invoice numbers.
In this scenario, you should reset the invoice in SageAPA and amend the invoice number to something unique. Once you export and sync the invoice again the invoice will populate in Sage 100 as it no longer views it as a duplicate.