Kaplan exam kit

Appler is considering the relevant cash flows involved in a short – term decision . An important client has asked for the minimum price for the processing of a compound . The compound involves the followings:

Material A: Appler needs 500kg of material for the compound but has 200kg in stock present.The stock items were bought 3 months ago for 5kg but have suffered 10% shrinkage since that date . Material A is not regularly used in the business and would have to be disposed of at a cost to Appler of $400 in total . The current purchase price of material A is $6.25/kg

Material B : Appler needs 800kg of material B and has this in stock as it is regularly needed.The stock was bought 2 months ago for $4/kg although it can now be bought at $3.75/kg due to seasonal nature . Processinf energy costs would be $200 and supervisor says he would allocate $150 of his weekly salary to the job in the company’s job costing system.
BASE ON THE SCENARIO INFORMATION , what is the total cost of material A and B to be built into the minimum price calculation?
Solution :
Mat A :
Stock saved disposal costs = ($400)
Bought 300kg @ $6.25/kg = 1875
Net cost = $1,475
Mat B:
Replacement costs needed
800kg@ $3.75/kg = $3000
Total = $1,475 + $3000 = $4,475.

My question : suppose we do not have 200kg but since the questions says we have it but needed to be disposed at a cost to us ($400) since its not regularly need . Can we complete the processing of the compound with just only 300kg when 500kg was required ? we had sold 200kg, now we have the cash . Why didn’t buy to replace it and make up the required 500kg for the processing ? We only bought 300kg and remove the costs of buying 200kg : meaning we needed just only 300kg for the actual processing and not 500kg.

Costs of material A and B should be (400) + $3,125(500kg@ 6$6.25)+ 800kg@ $3.75= $5725.

I wait your response maybe my idea of the scenario is totally wrong.
Thanks .