Learn Finnish: Syötin myyntiautomaattiin ison setelin, koska halusin vaihtorahaa.

The adessive of price is used when you exchange money for something, like in Ostin sen isolla setelillä (I bought it with a large banknote). Here, the banknote isn't the price being paid for an item; it's the direct physical object you are inserting into a machine, so it takes the accusative case instead.

Why is myyntiautomaattiin in the illative case?

The verb syöttää literally means to feed. When you feed something into a machine (like money, paper, or data), Finnish uses the illative case to show the direction of the input.

Why is ison setelin in the accusative case (which looks like the genitive here) rather than the partitive?

You are inserting one whole, specific banknote and completing the action. Because the object is a single, countable item and the action is bound to be completed, it triggers the total object case.

Why does vaihtorahaa take the partitive case?

Vaihtoraha (change) is being treated as an uncountable mass noun here. You wanted some undefined amount of change, and the verb haluta (to want) naturally takes the partitive for uncountable or undefined quantities.