Short answer... It depends! Trial by combat was not uniform throughout medieval Europe, so different norms were dictated in different areas. Champions weren't always allowed, and even when they were payment was sometimes strictly forbidden, but one of the most prominent places where hiring of champions was common, and indeed, expected was in early Norman England, where trial by combat was one way in which property disputes were handled, via "writ of right". In sum, if two people disagreed on who owned a piece of land, one made the 'writ of right', becoming the plaintiff (demandant), against the defendant (tenant). If there were records to prove who was correct, the matter was easy enough to settle, but in the event of a failure to prove ownership by either party, combat was a means of settling it.

Now, in theory, you weren't supposed to be using a hired sword for this, but the demandant was not supposed to fight. It was expected that he would use a champion, and the champion was supposed to be a witness to the claim. In reality, they were generally paid fighters, and the practice was entirely non-objectionable even if against the spirit of the law, as there seem to be no records actually making a fuss about it. The tenant could, if they chose, defend their claim themselves, but they too could appoint a champion, again, usually someone paid for their service. Both fighters would swear oaths as to the belief that their cause was right, but it was going through the motions.

The fight ended one of three ways: One person died, one person cried "craven", or the tenant won by default if the fight lasted until nightfall without winner. Obviously, no one wants to lose, but for a professional Champion, it didn't simply dent his reputation. He would be fined for perjury, and declared infamous, meaning he was out of a job permanently and could no longer be a champion.

As such, fighters with a reputation were invariably undefeated, and they could attract a high premium. This makes for an interesting way to analyze these fights. Peeter Leeson, an economic historian, offers the interesting thesis that the process offered a rational way to allocate land rights through a "violent auction", wherein two parties would be bidding for the better fighter. The cases of "writ of right" that went to combat were, by definition, those without evidence to support either case, so Leeson sees this as a way to ensure that the person more desirous of the property would win it, as they are likely to be willing to bid more for the better fighter. Not a guarantee of victory, but pushing the odds in their favor. It is only an analytical lens, of course, and not how people at the time would have characterized what was nominally "Gods will", but an interesting way to think of the process.

For further reading, see:

Trial by Battle by Leeson, Peter. in Journal of Legal Analysis 2011 vol: 3 (1)

Hired Champions by Russell, M.J. in The American Journal of Legal History 1959 vol: 3 (3) pp: 242-259

Trial by Battle Procedure in Writs of Right and Criminal Appeals by Russell, M.J. in Tijdschrift voor Rechtsgeschiedenis 1983 vol: 51