Is what an employee earns considered unlawful gain if he uses a mode of transportation that is cheaper than what the company allocated for transportation allowance, and he writes the maximum specified amount in the form?

It is not permissible for an employee to use transportation at a low cost and then record a higher price on the form than what they paid, taking the difference for themselves. This is because it is considered lying and deception, and the company entrusts the employee to record the true amount they spent.