Am I entitled to claim the agreed-upon compensation after my brother failed to pay the agreed-upon amount on time?

The question comprises two matters: First, it concerns the sale of a clinic device owned by the questioner without his knowledge or authorization by someone acting on his behalf at the clinic. This sale is void because it is an unauthorized transaction (فضولي) and a sale of what one does not own. Therefore, the buyer must return the device to the questioner and reclaim his money from the seller. The questioner then has the right to sell it, or he may ratify the initial sale.

Second, the matter relates to the questioner's agreement with the other party to increase the monetary amount in case of delayed payment. This agreement is forbidden because it is explicit usury (riba), and the Qur'an was revealed prohibiting usury.