Which is more deserving: paying off the usurious loan now that the money is available, or postponing the payment to save enough money to return to live in a Muslim country?

The situation described by the questioner is unclear. How will the duration of stay be prolonged to save the return fare if the principal of the loan is repaid immediately to rid oneself of usury? The installment payments will provide him with the required amount quickly. If accelerating the repayment will prolong his stay, and delaying it will enable him to depart sooner, then he is not sinful for not accelerating it. It is not a condition of repentance to hasten the payment of bank obligations, unless the acceleration leads to the cancellation of the usurious interest.