Wow, so many suggestions... If only someone bothered to stop and think whether the requirement makes any sense.
Anees, kindly go back to your SD consultant and explain that the billing document conditions must not (and cannot) be manipulated in such way. Pricing conditions need to be determined in the preceding document. It's actually very alarming that someone gave an ABAPer such requirement (unless I misunderstood it completely). You'll find many posts on SCN confirming that this is not the right approach and is not feasible.
P.S. Pinging TW Typewriter to look at this in case I'm missing someting.