"Surely you weren't expecting to be rewarded?"
"No. But Maestor, .....please don't call me Shirley."
Surely, copying old decaying manuscripts is a promotion (possibly multiple promotions) over scrubbing the bedpans of and delivering meals to invalids, right? It's actually work vital to the library. Why be so glum about it?
He's already found the information about the dragon glass and the cache of it beneath Dragonstone, so he's (at least partially) fulfilled his purpose for going to Oldtown in the first place. And how much do you want to bet that he finds some useful information in the scrolls he was tasked with copying?
I think the Sam-as-comic-relief theme could probably be toned down a bit. He ought to have been enthusiastic about the new job Professor Slughorn gave him, not depressed.