We use "будет" here because in this case we are talking about the future. You can compare: If I have time, I will help. (Conditional 1) Если у меня будет время (Future Simple), я помогу (Future Simple). If I have time, I always help. (Conditional 0) Если у меня есть время (Present Simple), я всегда помогаю (Present Simple). The difference is that you never use Future Simple after "if" in the English lanquage, but in Russian we do when we talk about the future.

Well, for some reason we don't like saying and don't say the word «иметь» (to have). If you say «Если я буду иметь время» (if I will have the time) this would be grammatically and logically correct, but it would be really weird hearing. We only say the word «иметь» (to have) in set expressions or when it's neseserily (no other way). That must be very weird to you because the word «быть» actually means «to be». Just like with articles (are, is): we have the two words (есть, являться) which fit the English (are, is), but we don't use it unneseserly. «Есть» is also used to say that you have something. I'm not sure, but the phrase, say «I have a dog» (у меня есть собака) literally meaning «At me there is a dog» or something like that. :) In this case «быть» is used in the past and future, «есть» in the present: - У меня есть время. – I have the time. - У меря было время. – I had the time. - У меня будет время. – I will have the time. - Если бы у меря было время. – If I would have had the time. - Если у меня будет время. – If I will have the time. P.S. I don't think it will be very difficult to learn how to use the word «быть» in such cases. It's just going to take you a while. Wanna practice that or something, – write to me. ;)

"if I WILL HAVE a time" can be literally translated as "если я БУДУ ИМЕТЬ время". In the passive voice this sentence has the following form: "если у меня БУДЕТ ИМЕТСЯ время". The reflexive verb "иметься (to have)" is obvious here and can be omitted.