Is it permissible to take financial compensation (a profit of 2-3 dinars) from the office when purchasing spare parts or devices for them, by buying them at a lower price than the market and then selling them to them at a higher price?

If you are an agent for the office in purchasing, then it is not permissible for you to increase the price beyond what you paid for the tools. However, if you are buying for yourself and selling to the office, then it is permissible for you to sell them at the agreed-upon price, but you cannot buy them from yourself for the office unless you are authorized. The apparent understanding is that you are an agent for the office, and the guarantee is on the office after the purchase.