Two thoughts: First, if you were only able to spend nineteen of the twenty dollars, is it possible that Apple nicked you for a dollar, meaning that Keith actually inadvertantly ended up paying for the song after all? Is there any way to verify this?
Second, is it possible for iTunes to carry more than one Apple account? That is, if Keith had already purchased a few songs on his own, with one Apple ID, and then you gave him your Apple ID, can he use both at the same time? If he had more than one Mac he probably could, simply by storing each Apple ID on a differetn computer and making the other one of the three allowed sharers, but would it work with just one Mac?