Is striving to increase the selling price of one’s brother's house from (11,000) to (11,400) by falsely claiming the existence of a second buyer at a higher price considered participation in lying, and is it obligatory to inform the buyer of this or to donate the extra amount as charity?

What the questioner indicated to his brother is a lie and forbidden deception. It falls under the religiously prohibited practice of najsh. Both of them must repent. The deceived buyer has the option to annul the sale or to take the difference if he was unaware and the deception was significant. As for the questioner, he is not obliged to return the extra amount or to give it in charity. Rather, he is obliged to advise his brother to forgive the buyer or to return the extra amount to him.