How can the human being be "Mosayyar" (bound to do certain acts) and "Mokhayyar" (free to do what he wants)?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

We cannot say that the person is either Mussayar (bound to do some definite acts) or Mukhayar (free to do what he wants). In fact, the person is both Mukhayar and Mussayar. He is prepared to do what he was created for. As for his being Mukhayar, it is because Allah has granted him a mind, an ability to hear, see and perceive. He has given him a will.
So, the person knows the good, the bad, the harmful, the useful, what is suitable for him and what is not and he can differentiate between these things. So, he can choose that which suits him better and leave that which does not suit him. The rulings of Shari'a address him whether they are commands or prohibitions; he gets rewards for good deeds and punishment for bad deeds. Allah Says (interpretation of meaning): {Verily, We have created man from Nutfah drops of mixed semen (discharge of man and woman), in order to try him, so We made him hearer, seer. Verily, We showed him the way, whether he be grateful or ungrateful.} [76: 2-3].
He also Says: {And by Nafs (Adam or a person or a soul, etc.), and Him Who perfected him in proportion; Then He showed him what is wrong for him and what is right for him; Indeed he succeeds who purifies his ownself (i.e. obeys and performs all that Allâh ordered, by following the true Faith of Islâmic Monotheism and by doing righteous good deeds). And indeed he fails who corrupts his ownself (i.e. disobeys what Allâh has ordered by rejecting the true Faith of Islâmic Monotheism or by following polytheism, etc. or by doing every kind of evil wicked deeds).} [91: 7-10].
On the other hand, at the same time a person is 'Musayyar' that means his actions cannot be free from the Might and Will of Allah.
Allah Says (interpretation of meaning): {And you will not, unless (it be) that Allâh wills, the Lord of the 'Alamîn (mankind, jinns and all that exists).} [81: 29].
The Prophet (Sallallahu Alaihi wa Sallam) said: "The first thing which Allah Created was the pen. He Commanded it to write. If asked: 'what should I write?' He Said: "Write the Maqadeer (decrees) of people till the Day of Judgment" . Imams Abu Dawood and Tirmizi reported it and Imam Tirmizi graded it sound. There are many verses and Hadith that confirm this fact.
Therefore, a human being is assisted to do what he has been created. The Prophet (Sallallahu Alaihi wa Sallam) was asked about the matter of decree and the actions of people whether it is predestined or not? The Prophet (Sallallahu Alaihi wa Sallam) said: "Do perform good deeds, for everyone is facilitated in that for which he has been created …" . [Sahih Muslim].
Allah knows best.