Praise be to Allah.
It is not permissible for you to make use of this money except in accordance with the conditions stipulated by those who are in charge of your work, which is buying a computer and using it as they have stipulated to update your engineering skills and so on. It is not permissible for you to buy the computer and give the invoice to the company, then return the computer to the store and take its price, as your colleagues have done. That is for the following reasons:
This is contrary to the conditions on the basis of which you took this money, and it is not permissible, because any condition that does not permit something prohibited or prohibit something permissible must be fulfilled, because the Prophet (blessings and peace of Allah be upon him) said: “The Muslims are bound by their conditions.” Narrated by Abu Dawod, 3594; classed as saheeh by al-Albaani in Saheeh Abi Dawood.
This is regarded as deceit, lying and using tricks to consume people’s wealth unlawfully, because you are making them think that you have bought the computer when in fact you have returned it and taken its price. Deceit and lying are both haraam according to the laws of Allah, may He be glorified.
It is proven that the Messenger of Allah (blessings and peace of Allah be upon him) said: “It is not permissible (to take) the wealth of a man unless he gives it willingly.”
Narrated by Ahmad, 20172); classed as saheeh by al-Albaani in Saheeh al-Jaami‘, 7662.
It is certain that if those who are in charge of the company knew that you had taken this money and not bought a computer with it, they would not approve of that.
In most cases the company only stipulates that their employees should buy these computers for the benefit of the company, by raising the standards of the employees, updating the computers that they use, and other things that will benefit the company and its productivity and efficiency. This is like what is called in sharee‘ah a gift given with the expectation of some return, which is when one person gives a gift to another in the hope that he will compensate him for it, which is a kind of gift with conditions or a gift that brings a reward. In the case of this kind of gift, if the giver does not get what he intended of returns or the recipient does not adhere to the conditions stipulated by the giver, the giver has the right to take back his gift. Ibn ‘Aabideen (may Allah have mercy on him) said in his Haashiyah (5/710): A woman gave up her mahr to her husband on condition that he take her for Hajj, but he did not take her for Hajj. Muhammad ibn Muqaatil said: She may take back her mahr, because consent to give the gift was on condition that there be some compensation; if there is no compensation there is no consent, and the gift is not valid if there is no consent. End quote.
Based on that, it is not permissible to make use of this money except in the manner stipulated by the company.
And Allah knows best.