yes you cant just not go and spend the money on whatever you choose if people have donated for this trip
and no deposits are not usually refundable so it does sound like you probably misunderstood.
the point of a deposit is to hold a place for you, and ensure that they dont get people just not turning up or not paying
bit pointless asking for a desposit if you can just ask for it back ...
what evidence do you have? if its just her word of mouth then she will just say you misunderstood - you would need it in writing