Modification for popup error at document level.

Dear Experts,

help with this what to do exactly and where? And what is the use of exit 'U101'

when user clicks on “Post” button for the FI document then exit:

• Looks for vendor account line in the document:
• If yes, check if vendor is submitted to withholding tax:
• LFBW- WT_SUBJCT= X
• If yes, check that BSEG-QSSHB is <> 0
• If yes result = TRUE
• Else result = FALSE
• And then it must trigger the validation message ZFI007.