You cannot use a future tense in a time clause.
"...before I`ve had a chance.." -is a time clause and the verb is in the present perfect simple. Before is a time adverb which means previous to that moment. In your main clause I`m leaving is in the present continuous tense and this tense is used with a future meaning here.
You need to use the present perfect in some cases when you need to show that one action can only take place after another has been completed, as in: "As soon as I've eaten this meal, I won't feel hungry any longer."
Does it help?