What is the ruling on a person demanding judicially from someone the expenses of an 'Umrah that a friend paid for him on a promise of repayment?

If two people agree that one of them will pay for the costs of 'Umrah this time, and the other will pay for the costs of 'Umrah next time, this is considered a فاسد (corrupt/invalid) commutative contract due to the unknown nature of the عوض (consideration/counter-value) within it, as the total cost of the two trips is unknown. Consequently, if the contract is فاسد, the questioner has the right to reclaim only what he spent on his friend for this trip. He does not have the right to demand that his friend pay for his outing on the second occasion as they had agreed. He may sue him if he finds no other recourse.