Is it haram if i add on the price and still recieve my payment for my service?

I told a sand dealer that i will be finding customers for him ,he told me the price he sells at and the amount he will give me for each sale we had an agreement.I found out that people buy sand at a higher price that he sells so i added on the price of the sand when talking to customers but lesser than the higher amount they used to buy at eghe sells at 1500 some people buy at 1750 so i told someone l600. Is this haram