Is money earned from selling a plot of land at a price higher than what the seller specified permissible or impermissible, taking into consideration that the broker's commission under local law is fixed at 3%?

If the landowner asks you to sell a meter for ten dinars, and whatever exceeds that amount is yours, then it is permissible for you to sell it for more, and the surplus is lawful for you. The law that specifies the broker's profit is irrelevant as long as there is mutual consent between you and the landowner. This is because brokerage is a transaction permissible by Islamic law, and its fee is determined by agreement and mutual consent.