Don't go below the purchase price ( vote for it )

I want this function let's say a product is purchased for 20$ and sell price is 23$ now the seller wants to give fix discount upto 3$ you are safe but more than that will be less since he don't know the purchase price there for i was wondering of the discount has a limit like don't go below the purchase price,
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***** Please All other members vote and follow this post.*******