Is it permissible for me to accept money or a gift from a supplier with whom a contract has been made, knowing that the final price was agreed upon by the company's manager?

It is not permissible for an employee to ask for or accept gifts from suppliers unless authorized by the employer, because "gifts to workers are ill-gotten gains" (ghulul). Therefore, whatever is given to an employee due to their obligatory work falls under this prohibition. The Prophet, peace and blessings be upon him, once reproached a collector of zakat for accepting a gift, clarifying that whoever takes anything from it will bring it forth on the Day of Judgment.