You ask a friend and they say "maybe", then make sure you ask to solidify plans by such and such a date. Say ok, well, we'll touch base on Wednesday about the weekend. Don't leave it open ended.

A friend asks you vaguely, then you are under NO obligation whatsoever to leave your plans open. I've been burned too many times by this. I won't block out time for someone tentatively unless it was someone really important like my sister flying in from the coast or something like that.