My two fishflakes of opinion...
It seems somehow...inethical to buy a fish knowing you'll need to be rid of it. In my most humble of opinions, if you're going to buy him and know you will upgrade to a tank sufficient for him, that's terrific (I did the same for my clown loach, knowing that we're buying a bigger tank this Christmas that will hold him). But not having that commitment ahead of time, and simply saying that you can then offload him when he becomes an inconvenience seems somehow wrong.
Perhaps you should start off with something that will suit the tank?
Again, just my opinion. Not an insult, not a flame, just my two brine shrimp pellets. Take it or leave it, it's up to you.