I wanted to know what would be the psak in a case where a baal simcha who specifically ordered home baked fresh foods for two different main course dishes in the end was provided by the caterer with only one of the dishes, and it was store bought. If he already paid 2/3 of the catering contract, how much should the caterer return to him (if he even has to return something)? Is there room for receiving all of his money back because of the ogmas nefesh caused to the baal simcha? After all, his guests only had one dish to choose from.
It is certainly possible that this will be considered a mekach ta’us, meaning that you did not get what you ordered, and therefore you have a right to void the transaction. This is particularly the case if you emphasized that you want two types of freshly baked foods.
The fact that you used the food provided is not proof that you validate the transaction, because you had no choice in the matter (see Pischei Teshuvah 232:1; Aruch Hashulchan 4).
However, in this case, because the foods he provided were eaten, you will have to pay the price of the provided foods.
Because the contract probably includes other matters aside from the baked foods you mention, the final amount you have to pay will be a little complex to calculate, and will be simplest to reach a compromise with the caterer.
If you don’t manage to reach a compromise, you don’t have to pay the remaining amount, but you will have to go to a beis din (or to an agreed borer) to ascertain how much you have to pay.
Although the ogmas nefesh is of course regrettable, it does not lead to any financial ramification.