Redsnapper: though the fart has mass(or in this case we are talking about weight), the argument is that it is lighter than air, ie it buoys you up a bit. Therefore, when you release it, your body actually gets heavier. This of course assumes that the fart inside your body is at atmoshperic pressure. If it isn fact compressed, it may in fact be heavier than air of course, in which case the situation is reversed. And if any portion of it is actually dissolved in anything else sloshing about in there, it will be heavier still.
This is not a simple questionb, and needs saome serious research.