How is his tooth money increase if he had to loose a tooth to get it? Seems to me he's breaking even and none of the money belongs to his imaginary friend.
Tell him the tooth fairy already took out the money for the tithing for him.
It was decided in the Heavenly Council Meeting of Imaginary Beings that the tooth fairy and Santa have to prepay to have membership in the Imaginary Committee of Imaginary Beings.
Better yet, put the tooth itself in the tithing envelope with a note that says, "We're getting Billy in the habit of giving his teeth to the church for when they might have gold fillings the church can take to build temples."
I'm not a parent but I would guess that helping a child figure out on their own that the tooth fairy is not real might be a great thing for them to learn.