Hi,
Hopefully you already tackled this in the 2.0 API, but currently there seems to be no way to find invoices by reference, only by FA’s internal entity id.
This means that to sync between two invoice systems that are only linked by the invoice reference, you have to get a list of all invoices in a given period and trawl them for their references.
That’s rather sucky. You can just try to create the invoice and get a 400 Bad Request (I think there’s a better status code to indicate the resource already exists…) but it feels bad.
Cheers
Marc