Regarding BADI FQST_CIN_WITHITEM

I am trying to implement this BADI ( BADI FQST_CIN_WITHITEM ) in f-43 tcode I found out this BADI by putting break point in CL_EXITHANDLER it is triggering while simulating in this Tcode but when I am trying to implement this its not triggering.

The requirement is the system calculated TDS when withholding tax base amount becomes greater than 50000 but we want for particular vendor and taxcodes and for the individual document the amount is greater or equal to 20000 and total amount for the particular vendor and taxcodes is less than 50000 then I have to give an error message.

Can any one suggest me the solution? Its urgent.