While pushing invoice data into Saasu, you should go with either approach below:
#1: Let the Saasu decide the invoice number (recommended)
This is the recommended approach. You can do this by providing the <Auto Number> from the API. However, please make sure that you escape both < and >. For example:
<tasks xmlns:xsd="https://www.w3.org/2001/XMLSchema" xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"> <insertInvoice emailToContact="true"> <invoice uid="0"> ... <invoiceNumber><Auto Number></invoiceNumber>
#2: Let the source system decide the invoice number
The source system must keep track of the latest running number and the number must be unique.