User Exit to Release Billing if Selling Price lower than Cost

Dear All,

I have a problem with user who always post selling price lower than cost. Example: Material A is cost 4$, and selling price posted 3$. I want to prevent this activity occur again & again by using enhancement [If selling price is lower than cost, system will show a message].

For instance, i user A enter billing which selling price<Cost, then system doesn't allow to save till user B approve.

Regards,

Ratana