relevant costing

Dear sir,
I have some issues concerning a relevant costing question(The question is actually from dec 2014 question 3 )

Wood: 50 m at $8.20 per m.
NOTES:This type of wood is regularly used and costs $8.20 per m. However, the company’s current supplier’s earliest delivery time for the wood is in three week time. An alternative supplier could deliver immediately but they would charge $8.50 per m. HL already has 500 m in inventory but 480 m is needed to complete other existing orders in the next two weeks. The remaining 20 m is not going to be needed until four weeks time.

-The answer is (20 x 8.20)+(30 x 8.50)
-In your lectures(revision kit live), you have said that “there is 20 in inventory and we need to replace it”
– Why we should replace it? It is already in inventory we just have take it and use it. According to me, it is a sunk cost because this (20 x 8.20) has already been incurred.

-Could you please clarify this point. I have some difficulties understanding the logic

Thanks in advance.