Ruling on the seller taking an excess over the agreed price

This is a message we received from Iraq, from the Salah al-Din Governorate, from the questioner Salem Mustafa Salem. He says in his message: I sold an agricultural crop in bulk to three grocers, and they were supposed to pay me the price in one lump sum, but they paid me each one separately. When I got in the car and counted the money, I found an excess. When I told the driver, he did not care to verify it, and if he had gone back, I would have given them the excess. So I ate the excess. Is it forbidden or permissible?
Answer: Yes, you must return the excess, and it is not permissible for you to eat it. If they gave you the price and the price was more, they made a mistake, so you must return the excess to them if you know them. If you do not know them and knowledge of them has passed you after your best effort, you give it in charity on their behalf to the poor and the needy. But if you know them, you must return it to them, either by yourself or by a trustworthy messenger who takes it to them. Yes.
The presenter: May Allah reward you with good. I mean: he must return the excess?
The Shaykh: Yes.
The presenter: May Allah reward you.
The Shaykh: Because it came to him without right, by their mistake. Yes.