Future tense in if-clauses

We normally use a present tense with if and most other conjunctions to refer to the future.

We will call you if we have time. (NOT We will call you if we will have time.)
I will call you when dinner is ready. (NOT I will call you when dinner will be ready.)

However, a future tense can be used in if-clauses to talk about later results rather than conditions.

Compare:

I will give $100 if you quit smoking and drinking. (condition)
I will give $100 if it will help you to seek treatment for your addiction. (Here the help is a result – it follows the gift of money.)

Should in if-clauses

We can suggest that something is very unlikely by using should in the if-clause.

If you should meet Peter, tell him that he owes me $100. (= You are not very likely to meet Peter, but if you do meet him, tell him that he owes me money.)

If…happen to has a similar meaning.

If you happen to pass the market, perhaps you could buy some fish. = If you should pass the market, perhaps you could buy some fish.

Note that should and happen to can be used together.

If you should happen to have time, give me a call.