Vous pourrez partir, si vous avez fini vos devoirs.
(You will be able to leave, if you have finished your homework.)
This strikes me as very peculiar. It seems that this would normally be phrased as either, You will be able to leave, when you have finished your homework; or You are able to leave if you have finished your homework.
Here's the rub. In the original sentence, although it's referring to being able to leave in the future, it's ruling out the possibility that you still have time to finish your homework.
So, in trying to come up with a si clause that includes passé composé and future tense that didn't seem peculiar, I came to the conclusion that you would only use this particular construction if it was the case that you were referring to a future event and something that, if it happened, had to have already happened. For example:
The good seats are all sold out. You will sit up front, (only) if you have (already) purchased your ticket.
And yes, passé composé means it already happened, but as I see it, in order for the construction to be appropriate, it needs to be ruling out the possibility of the condition also being met at some other point in time prior to the future event.
Is this conclusion correct?
If this is the case, I think knowing when one might use a particular construction is just as important as knowing that it exists. (And I think the ex. in the lesson is a weak one.)