What is the ruling on a real estate agent selling to his client at a price higher than requested, and keeping the excess for himself?

What this person is doing falls under forbidden lying, as the Prophet, peace and blessings be upon him, said: "Beware of lying, for lying leads to immorality, and immorality leads to the Hellfire. And a man continues to lie and search for lies until he is written down as a liar with Allah."

The sin increases if the lying is in buying and selling, as the Prophet, peace and blessings be upon him, said: "Three types of people Allah will not speak to, nor look at, and for them is a painful torment... The one who lets his garment hang below his ankles, the one who reminds others of his favors, and the one who sells his goods by taking a false oath."

Therefore, it is obligatory for him to repent to Allah and abandon this habit.