All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If your employer conditions that the amount of rent is the actual amount with which you rent the accommodation, then it is not permissible to cheat the employer by asking the owner of the house to write a rent amount that is not true so that you save a sum from it, whether you intend to use it for upgrading the house or for some other reasons, unless your employer authorizes you to do so because this is cheating and deceit and contravening the terms and conditions on which the contract was concluded with the employer. All this is not among the moral conduct of the Muslims. The Prophet said: "The Muslims must fulfill their conditions." [Abu Daawood]
Also, Al-Qaasim Ibn Muhammad Ibn Abi Bakr As-Siddeeq said: “I just found the people (the Prophet's Companions ) fulfilling their conditions regarding their money and what they were given.” [Ahmad]
However, if the employer does not condition this; rather, he allocates an amount of money for the employee to benefit from it in renting an accommodation whether he spends it entirely on rent or spends only some of it, then, in this case, it is permissible for you to do what you mentioned.
However, it appears that the situation is the first case, which is that your employer does not approve of what you mentioned. Therefore, you may inform your employer of what you intend to do, and if he gives you permission, then you may do so, otherwise no.
For more benefit, please refer to Fataawa 92207 and 90773.
Allaah Knows best.