Math question

When Moze & Addie are in the hotel room together the first night, Moze says, "I owe you $103.72," and she corrects, "74 cents." The next day Addie interrupts Moze's scam and gets the mark to pay $12 for the bible instead of $8. Later, in the car, she says, "You owe me $85.74." Why did the amount go down by $18? I would understand if it went down by $4 since that's the extra amount she managed to get out of the swindle, but why $18?