"Until then" suggests that you have made plans for something to happen in the future - meeting or speaking. With "Goodbye" it is a little more flexible (and formal). It doesn't speak about the future so can be used whether you are going to see/speak again or not.

Goodbye is what someone says when leaving. Till (short for untill) then can be used the same way, but is more colloquial and regional (here in America most people don't say that).