What is the religious ruling on an employee's claim that he sent a fax on a previous date and falsified its date to benefit from old prices, and what should be said to him?

What your friend did is a lie and an unlawful taking of people's money, due to the Almighty's saying: (So shun the word of falsehood), and due to the Prophet's (peace be upon him) saying: "Can a believer be a liar? He said: No."

He must repent to Allah by regretting and resolving not to return to it, and by returning the right to its owner by returning the goods and reclaiming the price, unless the supplier becomes aware of the truth of the matter and forgives him.