Charcoal, 65 bytes

Ｆθ«≔⁰ζＦ±§ι¹«§≔ι¹¦⁰≔⊟⮌Φθ∧‹⁰§λ¹⁼§λ⁰§ι⁰η§≔η¹⊖§η¹≧⁺§η²ζ»⊞ιζ»Ｅθ⭆¹✂ι¹χ²

Try it online! Link is to verbose version of code. Explanation:

Ｆθ«

Loop over the input array.

≔⁰ζ

Start with a COGS of zero.

Ｆ±§ι¹«

If this is a sales transaction, then repeat for the unnegated quantity:

§≔ι¹¦⁰

Clear the quantity of the transaction. (This doesn't affect the loop as it's already taken a copy by this point.)

≔⊟⮌Φθ∧‹⁰§λ¹⁼§λ⁰§ι⁰η

Find a purchase transaction for this item which still has a quantity remaining.

§≔η¹⊖§η¹

Decrement the quantity remaining of the purchase.

≧⁺§η²ζ

Add the cost of the purchase to the COGS.

»⊞ιζ

Save the final COGS (if any) to the item.

»Ｅθ⭆¹✂ι¹χ²

Output just the remaining quantity or the COGS for each item as appropriate.

JavaScript (Node.js), 107 bytes

x=>x.map(y=([i,q],j)=>y[j]=q>0?[q,,]:[,(g=j=>q?x[j][-!y[j][0]]==i?g(j,--y[j][0],++q)+x[j][2]:g(j+1):0)(0)])

Try it online!

Generator

x=>x.map(y=([i,q],j)=>     // y stores refs to modify
  y[j]=q>0?[q,,]:          // purchase
  [,(g=j=>q?
    x[j][-!y[j][0]]==i?    // x[j][0]==i && y[j][0]
      g(j,--y[j][0],++q)+x[j][2]    // Buy at cost on index j
    :g(j+1):0)(0)])        // else try next

Crash y[j][0]=undefined[0] in case oversell