This is covered in the An New Hope: Junior Novelisation. In short, his reward (in the boxes we see on screen) is made up of precious metals that are evidently used in the repair of spaceships;
Stepping down to the hangar floor, Luke found Han and Chewbacca loading small boxes onto an armored military speeder. The boxes contained precious metals, the only form of currency that Han would accept from the Alliance. Han had insisted on the payment, even though the Rebels desperately needed the materials for repairing starships and equipment. Han appeared to be completely ignoring the activity of the Rebel flight crews and pilots.