Is the questioner sinful and obligated to return the amount he received from the gas station after the workers discovered a leak in his car's air conditioning, even though they did not discover the leak before charging the Freon for the first time?

If the worker was negligent and derelict in his duty by not inspecting the air conditioner before filling it with gas, then there is nothing wrong with you in reclaiming the money from him, because the benefit was not achieved. He is responsible for the value of the leaked gas due to his negligence. It is preferable to accept the worker's offer to fix the defect and refill it, or not to take the money from him as a precaution and out of piety. The manager deducting the amount from the worker is not the concern of the questioner.