This is covered in the [A New Hope: Junior Novelisation][1]. 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.

Moving down the canon scale, this is also backed up in the (alas, now non-canon) [Star Wars Radio Dramatization][2]:

> **Leia:** *Captain Solo may have no morals whatsoever, Commander, but he's right. BOth Luke and I promoised 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!***


  [1]: http://starwars.wikia.com/wiki/Star_Wars_Episode_IV:_A_New_Hope_%28junior_novelization%29
  [2]: http://en.wikipedia.org/wiki/Star_Wars_%28radio%29