@HDMI1
It's a bit more complicated than that but the first sentence is right.
The reward amount is that much because of the request.
I'm sure that reward is for A) Saving his daughter. B) Compensation for buying Kana. and then C) The request.
If he refuses, I'm sure he will still at least get good amount of the reward for saving his daughter...otherwise they would lose face as nobles for not rewarding a favor done to them.
@Sep10trion
That would actually be pretty dumb. First, They already owe him a favor due to saving their daughter. Second, he wasn't lying when he said he was broke. Third, the power behind having a noble family sponsor you (short sword with their family crest) is profound. So just offloading the girl who you spent all your money to buy, refusing twice that amount and a sponsorship for a favor already owed to you.....poor, poor decision. You'll make a great isekai MC!