There are only two suggestions I would offer, and they both deal with the possessive. In "towers weather-beaten walls", towers needs an apostrophe. The "Sorcerer's imp familiar" seems redundant, since you've already told me the imp belongs to the sorcerer. That tells me he is a familiar.

1)Ambar's arms quivered with fatigue as he gripped the ripple of limestone and inched his way higher.
Crit)I think I would leave out "with fatigue" because your next sentence conveys that by describing Ambar's breathing. How about "rippled limestone" and leave out the "of"? I know you like to use "as", but I think with a little more thought, this sentence would read better as two (and there's an example of a good way to use "as" ).

2)He clung there like a lizard to the towers weather-beaten walls, his breath coming in ragged gulps.
Crit)Maybe "lizard-like" and drop "a"? I also think "wall" would work better than "walls". I don't see how he could be clinging to more than one at a time.

3)Fool! He cursed himself for the hundredth time that day.
Crit)"Fool!" says the same thing as the next sentence. Is it necessary?

4)What had gotten into him, to think he could squeeze even a single bronze piece more out of Urtis. He’d known better than to haggle with the thin skinned old Sorcerer over the value of the scrolls. Once Urtis set the price it was final, no quibbling.
Crit)Is your first sentence a question? I try not to repeat myself by saying the same thing differently in a series of sentences. I see this here where you might be able to tighten your phrasing and still get your meaning across.

5)He should’ve bitten out his wagging tongue the instant the Sorcerer's imp familiar leapt to the back of his master’s chair and commenced whispering into his ear.
Crit)I feel that "familiar" is hinted at through the word "master". I would try dropping "familiar" and see if you still don't get your point across.

6)But no, he prattled on about the risks taken to find his worm-eaten treasures in the Rift. Ambar finally shut up when he realized they were no longer even listening.
Crit) This I like. It hints at things to come and starts us into asking questions we hope will be answered shortly. Like the rest, there are still places where the writing could be tightened and some words disgarded.

I realize that you're writing from Ambar's POV. So if any of my comments goes against how your character thinks or acts, I apologize. I'm a strong promoter of not using anymore words or descriptions than are necessary to tell a story. This is especially true in writing short stories where you must tell your tale in a fairly precise manners in a limited number of pages.

I agree to cut out "with fatigue", you already have shown us don't follow by telling us.

As far as "What had gotten into him, to think..."

I think this should be tightened up to a much shorter sentence. You follow it with "He'd known better than to haggle..." Saying the same thing twice IMO.

The whole last paragraph seems to go on about the haggling. I thing the last paragraph can be tightened up more.

Here it the revised first 13.

The "What had gotten into him . . ." line needs to end with a question mark.

The flashback bugged me some, because you took me away from the "cliff hanger" for a little too long, IMO . . . which could be fixed if you tighten it up, as Crystal recommended. Don't go on and on about how Ambar haggled--get right to the point of *why* doing so was foolish. Along which same lines: the " He should’ve bitten out his wagging tongue the instant . . ." sentence dragged on a bit, for me. Probably because you say "the *instant*," and then go on for what seems almost like another two sentences to describe this "instantaneous" thing. I exaggerate, but that's kinda the feel I got there.

And finally, you should go back through and change the past tense in the flashback to past perfect, i.e. "he'd prattled on" and "Ambar had finally shut up." That way, when I hit the "Ambar finally shut up" sentence, I'm not suddenly wondering whether he's been chatting with them while hanging onto the tower walls, or some such.