@mahtan
First they gave up a lot of money because it would be "useless" to them
Did you miss the part where they said that it troubles them? They were getting slandered.
One way they could ever use that much money is to start a new territory, which explained was that it doesn't guarantee success and that you have a lot to lose if you fail. It goes to say that starting a merchant company when you don't have the vision, connections, nor the skills wouldn't work out either. Even then, the issue with their reputation wouldn't be solved if they kept the money for themselves.
They only really want to serve under the MC, so the money is only useful for equipping themselves with.
The one asking for a request is a margrave, a much higher rank than he is. MC would unnecessarily draw the ire of other nobles if he were to ignore the hierarchy.
The margrave is acquainted with his master, generously rewarded him for bringing him back the margrave's assets, and I assume also has facilitated the inheritance of his master's assets. So MC kinda owes him.
Also, there's a very good reason to cull the undead in that forest, more than for the sake of the nobles and the feudal lord of that territory. It's also the church's responsibility, who he is tied to through his fiancee.
There's no reason to say no at all.
@zz2000
Silly question, but is MC in a position to refuse the requests that come his way?
As mentioned above, refusing this sort of quest from that certain person is more trouble than it's worth. He's knee-deep into politics, and his interests would have to align with the kingdom.
@Tanatama
MC is really smart, he just finds it hard to refuse from being pushed around. If you consider he was a modern person, there's a lot of behavioral variables as to why that is.
If we're to compare this to some modern society, it would be like being in a village in the middle of nowhere where everybody knows each other, and going against the status quo will risk you getting ostracized. You don't want to be ostracized when you're living in the middle of nowhere. The village would be the whole kingdom, and the residents would be the noble families and large factions.