Is it permissible to take a percentage (10-20%) of the value of goods by adjusting the purchase price in the invoice submitted to the company, with the approval of the purchasing coordinator and the manager, in return for the effort and time expended in the purchasing process which is carried out using the employee's personal cards and through their personal expertise?

If the purchasing coordinator and your manager are authorized to pay you a commission for your service, then there is no objection to it. Otherwise, their permission is not considered. The work system can be reviewed, and their authorities can be inquired about. If they are not authorized, then it is not permissible to take a commission by altering the original invoice. However, it is permissible to refrain from performing a service beyond your regular work unless the commission is paid.