. . . until he fixes it shows that the speaker is dealing with a real situation. We know this because the speaker is using the verb fix in the simple present in the indicative mood. There really is something broken, and from what we can gather, the repairman has asked for payment in advance. What we have here is the advice that the speaker is giving to the person who needs the repair work done.
. . . until he fixed it shows that the speaker is talking about an unreal situation, a hypothetical case. We know this because the speaker is using fix in the simple present subjunctive mood. Nothing is broken at this time, but the two people are discussing "what-if's," that is, what one or the other would do if such a situation were to develop.OK, here's the situation: I'm telling my friend about a contractor who has billed me for work on my basement. The work is complete, but during construction a window was damaged. My friend's advice: "I wouldn't pay him until he fixed it."
Why does my friend choose the past tense fixed instead of the present tense fixes? (As for the idea of the "simple present subjunctive mood", let's just say that google finds only two instances of this phrase: one is Firsten's and the other is a Spanish course outline.) Not because nothing is broken, but because my friend sees it as unlikely that the contractor will offer fix it. If, on the other hand, the contractor had already said he'd fix it, the simple present would be a more likely choice.
That's not to say that ...until he fixed it would be inappropriate for a completely hypothetical situation. Of course it would work there too. It's just not as limited as Firsten would have us believe.