It's kind of meaningless to have infinite money, seeing how he got it in exchange for stuff money can't buy. If we were realistic, he should watch his back as well. The country will struggle to operate with those payments draining the budget year after year. However, if those guys suddenly disappeared, were proclaimed traitors, etc, all the financial troubles would be gone. It doesn't really even need to be anything that dramatic with the MC being such a pushover. I could see somebody working for the king coming to renegotiate it, lamenting how the kingdom is doing so poorly, people are suffering, bla bla bla, and the MC would agree to have only 10% of the reward paid in the end. Or they just trick him otherwise since he's so gullible. They could even turn the population against them, spreading rumours the MC and his merry bunch are robbing the country's money.
He has zero dreams or ambitions, so I doubt he will even try to get himself involved in the research of the stuff they found.