Error -5002, Update quantities in purchase order? (INV1.Quantity) Line[2]

We are trying to create Invoices using the SDK tool for a clients interface. Everything goes ok, except at the point where we add the document, we get the following message:

Error -5002, Update quantities in purchase order? (INV1.Quantity) Line[2]

Any ideas? We have searched the SAP notes, but nothing is available. Are we missing anything?

We are running on Version 6.50.098

Thanks

Marios