adding discount details at the time of invoicing

hi all,

suppose there is a PO with 100 items(services), some of which have been invoiced. We have a SES (say,x), at the time of invoice the vendor agreed for a lumpsum discount, which is applicable only for the services in SES x. How can we implement this discount in the system?

while parking invoice we can show only 5% cash discount. If we change the discount in the PO header, the discount will be distributed all entry sheets, and as i already mentioned, some of the services have been invoiced already.

so how to incorporate the discount in the system?

thanks!