cost of goods sold formula

I am having trouble with a formula that would take the inventory that is left and subtract what is needed from the last purchases, then the next-to-last purchases for first-in-first-out or takes from the first purchase, then second purchase for last-in-first-out.

= IF((C15-C11)>0,((C10-(C15-C11))*B10),(C10*B10))

It works for that, but using goal seek to make the net income 0 from last-in-first-out on my attached sheet makes a negative number.
I need help finding a formula that will subtract from 5, then 4, then 3 for FIFO or 1, then 2, then 3 for LIFO and I am completely clueless.