I made a Niyath ( intention) that if I get a beneficial thing (job matter only), I will do some beneficial things for Islam. Now I received that expected thing. The question is that whether I have to do my Niyath or is it prohibited to do that Niyath in Islam, and, if so, what shall I do?

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

If you make a vow to Allah to do some charitable deeds in case of getting a beneficial job, then it is your duty to fulfill your vow as long as you received such a job and you are able to fulfill the vow. Otherwise, you will be sinful for the Prophetic Hadith: "Whoever vows that he will be obedient to Allah, should remain obedient to Him..." [ Bukhari ]
But if your intention is just to do some charitable deeds in case of getting a beneficial job, i.e., without making a real vow, then such an intention constitutes a good deed that one is advised to fulfill and to avoid giving the devil a chance to make you draw back.
Allah says: {Shaitân (Satan) threatens you with poverty and orders you to commit Fahshâ (evil deeds, illegal sexual intercourse, sins etc.); whereas Allâh promises you Forgiveness from Himself and Bounty, and Allâh is All-Sufficient for His creatures' needs, All-Knower.} [2:268]
This last condition does not constitute sin if you don't fulfill your intention although you will be rewarded for doing such a good deed.
Allah knows best.