What is the ruling on the action of one who is entrusted with purchasing an item at a specific price, but it turns out he purchased it for less than the price he stated?

The questioner: Peace be upon you.

The Shaykh: And upon you peace.

The questioner : Regarding trade, one of the owners of the houses gave me money and told me to buy tiles. He was buying the meter for one hundred and ninety qirsh, so I told him, "I will get it for you for one hundred and fifty." I went and got it; I found someone selling it for one hundred and twenty, and I charged the house owner one hundred and fifty. Is this permissible or not?

The Shaykh : It is permissible or not permissible. Did you inform the man who appointed you as his agent to buy that you bought it for one hundred and twenty and charged him one hundred and fifty?

The questioner : No

The Shaykh : Then it is not permissible.