Is the questioner entitled to take the difference between the agreed-upon selling price of the plot of land (14,250 dinars) and the price he paid (13,700 dinars)?

What you did between your brother and the land seller is considered brokerage, and it is permissible to take a fee for it from one or both parties to the contract, provided that the one from whom the fee is taken is aware and agrees. It is not permissible to deceive your brother and make him believe that the land price is higher than it is, so that you can take the difference for yourself without his permission and consent.