AGS Update to AP20 detail

I have a processflow that is adding a detail record to the AP20.  After the add takes place under some conditions I need to go back and update the record that was just added.  I am having a hard time identifying the record that was just added when there are multiple records.  I thought that I could get return variables from the previous AGS transaction such as and use that to identify the record that was added.  But I am not getting anything returned.

I do not see any other way to identify a detail record besides fc0 or fc1, etc...  I need to do this dynamically instead of referring to r0.

The add:
_PDL=&_TKN=AP20.1&_EVT=CHG&_RTN=DATA&_LFN=ALL&_TDS=IGNORE&FC=C&API-VENDOR=&API-INVOICE=&API-COMPANY=&LINE-FCr0=A&APD-DIST-COMPANYr0=&APD-DIS-ACCT-UNITr0=&APD-DIS-ACCOUNTr0=&APD-ACTIVITYr0=&APD-ACCT-CATEGORYr0=&APD-DIS-SUB-ACCTr0=0&APD-ORIG-TRAN-AMTr0=&_DELIM=%09&_OUT=XML&_EOT=TRUE

The update:
_PDL=&_TKN=AP20.1&_EVT=CHG&_RTN=DATA&_LFN=ALL&_TDS=IGNORE&FC=C&API-VENDOR=&API-INVOICE=&API-COMPANY=&LINE-FCr0=C&APD-DIST-COMPANYr0=&APD-DIS-ACCT-UNITr0=&APD-DIS-ACCOUNTr0=&APD-ACTIVITYr0=&APD-ACCT-CATEGORYr0=&APD-ORIG-TRAN-AMTr0=20&APD-ASSET-TEMPLATEr0=&APS-ITEM-QUANTITYr0=&APS-INSRV-DATEr0=&APS-PURCHASE-DATEr0=&_DELIM=%09&_OUT=XML&_EOT=TRUE

Any help would be greatly appreciated.