"if"

I'm a German learning English. I was told that you should'nt use "will" or "would" in an "if-sentence". However, in one of my lessons of my English book one person says to another: We are going to have a party tonight, that is if Mr. Jones won't mind.
Won't means "will not", doesn't it? Why is it still correct? I suppose it is, since it's a book on English language.
I'd like your answer. Thanks in advance.
Werner

It's correct.

To me, "doesn't" seems to fit slightly better in that sentence than "won't".

Are you learning British English or American English?

Yeah, I (speaking American English) would prefer "doesn't" over "won't" in that context.

My book is on British English, but it is a bit dated - as I am. Of late, I have been spending the winters in florida.

Werner,

It really should be said "We are going to have a party tonight, that is if Mr. Jones DOESN't mind;" or even better, "We are going to have a party tonight, PROVIDING Mr. Jones DOESN'T mind. " The use of "won't" sounds a bit vernacular.

Yeah, won't doesn't sound quite right- doesn't is more correct. Books are often full of grammatical mistakes.

How about these two sentences?

I'll call you if I'm late.
I'll call you if I'm going to be late.
I'll call you if I'll be late.

To me 1 is the most natural in the context where the person if already late as he makes a phonecall, and 2 is used when he is still not sure if he's going to be late or not (but thinks he will) as he calls, along with 3.