Need to change value for field SUMME in IDOC-INVOIC

Hi,

While creating invoice, EDI-IDOC is generated and i need to change the sign from '+' to '-' for field SUMME, based on field BSART from E1EDK01 segment.

Kindly let me know how to change the sign for field SUMME.

Can i write implicit enhancement in FM - IDOC_OUTPUT_INVOIC

Kindly guide.

Thanks,

Gopi