I have no personal experience with sailing a raft (though I do sail), but as it's getting on three weeks without an answer I'll give it a shot.
I see two primary concerns here: the stability of the sail on the raft and the overall stability of the craft.
To make the boat stable, the center of gravity has to be below the point on the mast directly over the edge of the boat when the boat is tipped as far as you want to let it go without tipping. The boat is all of five pounds, so how heavy and tall the mast is matters quite a bit; a sailboat has a good deal more weight to counterbalance the mast. You will be able to help a bit by leaning like in a real sailboat, but not as much. And the sail and boom add a non-negligible amount of weight higher up and towards the side of the boat. So you'll want a light mast, boom, and sail, preferably bottom-heavy.
After considering this for some time, I have come to the conclusion that your best bet is a windsurfing sail. That will also help some with the center of gravity problem when you're not actively using it because you can lay it down in the boat. It ought to be big enough to get you moving, though probably not as fast as a full sail. It should be relatively light, and you can tilt it so that its center of gravity is in the center of the boat. But you will have to hold it while you're going.
Now consider that an inflatable raft isn't going to have a solid spot to put the mast. How are you going to secure the mast to the raft? Honestly I don't have a good suggestion for this beyond rigging up an ugly frame somehow.