What was in those green metal boxes anyway?
From Star Wars Episode IV: A New Hope (junior novelization) non-canon:
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.
From Star Wars (radio dramatization) non-canon:
Leia: Captain Solo may have no morals whatsoever, Commander, but he's right. Both Luke and I promised him payment for his and Chewbacca's help in this matter.
Willard: Very well, I see I misjudged you, Solo
Han: I'll cry later. Right now, I'll settle for cash. In small, used notes if you've got 'em.
Willard: But this is a Rebel camp! We're hunted people, with very little Imperial currency among us!
Han: Oh. Well, your tech facilities must have stocks of precious metals.
Willard: Yes...
Han: They'll do.
Willard: But those are critical matériel! We need those metals for repairs, to keep our weapons and equipment functioning!