Protect Condition Value Column in VA01

I have a security problem on my sales orders: If somebody adds a line and enters the discount Z condition and some number on the value column, SAP updates the Order without going thru the regular checks of discount limit which it does on the amount column.

What I want is to put an if to check if the Amount column its 0, to delete the number on the condition value column, I have tried with ' ' but the value turns to cero and it does not show an error.

Here its the code:

IF xkomv-kbetr = 0.
        MOVE '' TO xkomv-kwert .
        MODIFY xkomv INDEX sy-tabix.
    ENDIF.

Regards,

Carlos

Edit: or if somebody knows how to make screen input = 0 (gray) for the Value Column that would be great.

Edited by: Carlos Salazar on Jul 9, 2010 4:09 PM