IPod App Question? ()()()()()()?

I recently got a $15 iTunes gift card, and after a while I only had $8 left. There was an in-app purchase for $10 which I made. The problem is, my iTunes account still says that it has $8 left, so I am worried that apple will make me pay for the full purchase seperately, instead of the $2. Has this happened/will it workfrom my iTunes account?