@Sternberg @Memi Except by forgoing his own system, he's still making the villagers dependent on him. If the amount of medals was too much, he could've only accepted a smaller portion and ordered the piano, instead of thinking "they had a good reason" and given it to them for free.
That just enforces, "If the reasoning is good, the chief will take care of it directly." Which he wanted to avoid having the villagers come to him for every little thing.
Additionally, the reward medal's value is based on the MC's own actions; if he had accepted the pool of medals or set a smaller percentage as "payment" for the piano, it helps stabilize the value and create a true economy for eventual currency in the village. Instead he now has to manually adjust and set value for goods procured outside of the village's means, which defeats the whole purpose of introducing currency; lowering his work load.
And remember, that was the whole point of introducing currency, to reduce his work load so he could focus on farming.