All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
The employee is considered an agent for the company, and it is not permissible for the agent to take anything in exchange for his work except with the knowledge and permission of his employer to do so.
Ibn Qudaamah said in Al-Mughni:
“Imaam Ahmad said in a narration by Muhanna, 'If a man is given a garment in order to sell it, and he did so, and the buyer gave him a handkerchief as a gift, then the handkerchief is for the owner of the garment.' Imaam Ahmad said this because the gift of the handkerchief resulted from the sale, so the handkerchief is an increase in the price, and an increase in the price in the sale session is added to the price.”
The Standing Committee was asked, “I entrusted someone to buy goods for me; its price is 5 pounds, for example, but the seller sold it to him for 4.5 pounds only, is he permitted to take the remaining 0.5 pound or not?”
Answer: “This is considered an agency (power of attorney), and the agent is not permitted to take anything from the money of the person who gave him agency except with his permission pursuant to the general evidence of the prohibition of taking a Muslim’s money except if he gives it out of his own proper will.”
Therefore, the amount that your friend is saving from the bill after convincing the other party is not permissible for him unless his employer authorizes him keep it.
For more benefit, please refer to fatwa 231187.
Allaah knows best.