"Alright but It's not a promise, okay?" means "좋아요, 하지만 약속하는 건 아니예요".
However, I could not get the situation completely because someone explicitly asked you out (데이트해요) and this is much more like yes or no question and if you say "it's not a promise (약속하는 건 아니예요 / 확정된 건 아니예요)", then the other will consider it as denial.
So, if you still want to date with him/her but do not want to confirm the schedule, then just say
"좋아요, 시간 한번 맞춰봐요(Alright, let's arrange the schedule for it later)" or "좋아요, 그런데 당장은 말구요(Okay, but not right now)"
Those are will be more natural and the other will not feel bad with them.