I was talking with this NPC and accidentally changed my faith with a single misclick. I then donated 100GP because I expected to be told how much to offer before donating. I only wanted to look at the option in case it was a "buy enchantment" or "give token roleplay amount" one, and it cost a considerable amount of gold.
Please give it an Are You Sure option saying what each dialog does and how much it costs.
It's the same for other Preacher NPCs as well.