Scrap and raw material

I have product item A, to get product 100 unit of A need 1 kg of raw material Item B, and this process it get scrap by-product item C kg.
Operations only one.
The quantity of scrap C. Not fixed may 10% to 50% depends on many reasons.
When create Work Order for Item A and qty is 200 unit , now i should transfer for manufacturing raw material Item B qty 2 kg.
Job Card for 200 unit is submit and scrap item is C and qty is 0.5 kg
When create manufacturing stock entry for 200 product A and 0.5kg of scrap C.
Why it not stop me as the scrap consuming 0.5kg of raw material and to add 200 unit of A need 2.5kg of raw materials.