Well, my thoughts had already been stated replies above me but I'd like to say other things.
War is very very expensive. Based on previous chapter, I'd say that 4000 gold coins are the annual income in the capital or something, which is huge. 100 gold coins might be 1/40 of it but for any adults here, you know that after substracted by everyday expenditures, you won't be left with much money. That's why lowering taxes works wonder to stimulate the economy because even if it's little compared to gross income, it is huge compared to disposable money after you spend it on food, housing, clothes, and other necessary expenses. You'd be able to buy other stuff that you didn't actually need to live, like artistic stuff, phones, which would increase the circulation of money and goods, which also translates to economic boom.
Anyway, then why the fuck would he just give away such large sum of money? 100 gold coins might as well equal to 10000 soldiers for a month. The MC is already very loyal and also very understanding. He could've just promised to reward them after the war is over.