how to get parent transaction id when PO return to vendor « Locked

Dear All

we need to integrate data with 3rd party system.

we complete PO receiving and return to vendor process in the 3rd party system,and then automatically sync data to Purchasing Cloud through FBDI by using webserices.

by now,we can sync PO receiving data successfully,but the "RETURN TO VENDOR" process is failed without populating the parent transaction id,the error show like "You must enter the parent transaction information for all receipt-based transactions".

so  how could we get the parent transaction id by webservices? or is there any other way to complete the "RETURN TO VENDOR" process without parent transaction info?