Will is a complicated word. Calling will VERB “the future tense” is beginner's grammar which will not stand up in many contexts.
- (In case you’re worried, I’m not going to open the contentious question of whether it’s permissible to call a periphrastic construction a ‘tense’.)
To begin with, it is not by a long shot the future tense; it is at most a future tense, alongside shall VERB, be going to VERB, and the simple present of VERB.
Next, will VERB does not necessarily mark a clause as having future reference. In many if .. then constructions (although not this one), it implies consequence, not subsequence, logical rather than temporal ‘following’:
If you add one to itself you will get three.
If he’s in Dallas he will be at the Ritz-Carleton.
In other cases will is employed in clauses which do have future reference, but the futurity is inherent in the proposition itself, not in the verbal construction. The if you will sit here is an instance: futurity is implied by for ten minutes, not by will, which is why the simple present is just as acceptable here.
In this particular case, will is used, in both if you will sit and I will tell, in its older sense of be willing.
If you are willing to wait here for ten minutes, I will* (consequential, not futurive!) be willing to tell the manager …
The same sense, with somewhat enhanced courtesy, may be expressed with a conditional past:
If you would sit here for ten minutes, I will tell the manager …