Is the commission taken by the head of accounting from the supplier permissible, given that the supplier's price is lower than others, and an agreement is reached with him after the company's approval, and on condition of quality?

If providing suppliers is part of your job duties at the company, then it is not permissible for you to take a commission from them, as this falls under prohibited gifts to employees. Furthermore, as you are an agent for the company, it is not permissible for you to take a commission without the permission of your principal. However, if providing suppliers is not part of your job duties and occurs outside your working hours, then there is no objection to you taking the commission.