If you do a Where-Used-List of ACGL_HEAD-SUMSOLL and check how the programs are using it, we see that it is adding the values (BSEG-WRBTR) of all DEBIT line items from BSEG (BSEG-SHKZG = 'S'). You already have the parent key for BSEG ... BUKRS, BELNR and GJAHR from the FIPP object key. You only have to sum up all Debit Line items (BSEG-SHKZG = 'S') to get the SUMSOLL.
So create the step before you are using it in the rule, calculate and pass back to WF.
Regards,
Modak