to validate inv. discount amount in sales line

Dear All,

I need to validate the invoice discount amount in sales line by entering the amount in the sales statistics page i.e. page opened by pressing F7 . but when i enter the discount amount in that page the value doesnot validate any code writtien in ONVALIDATE trigger of this field. how can i valiedate the inv. discount amount of sales line by validating the amount in sales statistics page.

thanks,

Suman