You could set up a court-like system where respected judges decided whether promises had been violated or not.
Which authority decides who's a respected judge? You not only have to defend the court against perception of bias but also about being actually biased because you pick the wrong people to be judges.
If the judges are on government payroll, can the politicians who control the government reduce the salary of the judges if the judges come to a conclusion that the government doesn't like?
There are already supreme courts with political powers which are paid from the public purse. It's not an easy problem to solve, but it is not unsolvable.
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