Is it considered usury for a brother to sell his brother's land to finance his marriage, then return the original amount with an additional ten thousand, and is it permissible to accept this offer?

The transaction mentioned combines agency and a loan. Your brother acts as an agent selling the land. The loan would be usurious (riba) and forbidden if the amount is repaid with a stipulated increase. It would be a good deed, rewarded, if there is no increase, and its reward is half the reward of charity. To avoid usury, you can sell the land to your brother with an increase in the deferred price. This is a "Tawwaruq" transaction, which has been permitted by the majority of jurists.