Thank you for your amendments, guys.
My manual says that there is a method using which it is in practice easier to define whether a Future tense must be used:
if "when" means "when exactly", i.e. a month, day, hour etc., and the logical accent can be made on "when", then a Future tense is used.
I don't know when he will come.
no idea when i will get paid.
if no stress on "when" can be made, then a Present tense is used.
I'll ask him about it when he comes. In no way is "when" stressed here.