Is it permissible for a student to take the remaining amount from an employee if the lab refuses to stamp the receipt due to the falsification of the amount?

The correct approach is to issue a new receipt with the actual cost of the analysis, stamped by the laboratory. The university then pays this amount, and the employee returns the remaining money to the questioner, thus ridding himself of ill-gotten gains and ensuring the questioner receives their rightful due. If the questioner is unable to obtain a stamped receipt and pay the cost from their own money, there is no sin upon them in reclaiming what they paid from the erring employee, as harm is to be removed.