As Su.ki stated they do sound a bit unnatural. І havе to lеavе now. І'll havе іmportant talks. (sometime later in the day sounding ,maybe that you have to prepare for). І havе to lеavе now. І am to havе іmportant talks. (sounds like it is sooner than later).

Once again, Danyel, it's difficult to say what the 'difference' is when you are giving us sentences which are unnatural uses of words and constructions. In fact, we would not use either of these forms in this situation. The 'will' future, when used in spoken English, is nearly always for decisions made at the moment of speaking, such as changes of mind, offers, suggestions and promises. We do not use this form for scheduled arrangements. Nor would we use the 'is to + infinitive' future. As I explained to you yesterday, this form is (a) formal (b) not used in spoken English (c) relatively rare. To be honest, I suggest you forget about this form altogether. As English speakers, we very rarely use this construction, and you do not need to use it at all. For scheduled events, such as meetings, we use the present simple, present continuous, or possibly the 'going to' future. To make the sentence more natural, I'm replacing 'talks' with 'a meeting', because that's what we'd be most likely to say in this situation, for example: 'I have to leave now. I have an important meeting.' or 'I have to leave now. I'm having an important meeting at 10 o'clock'. I hope that helps.