Is it permissible to benefit from an airline ticket sales agency owned by one's brother, provided that the agency repays an usurious loan specific to the brother? And how is zakat calculated for an airline ticket sales agency that buys seats and then sells them at a profit or loss?

It is not permissible to sell the agency while continuing to pay off an interest-based loan, because that involves cooperation in sin.

Zakat is obligatory on tickets or their monetary value if they reach the nisab (threshold) and a hawl (full lunar year) passes over them. The Zakat rate is 2.5% of their value. As for devices, furniture, and buildings, there is no Zakat due on them.