Regulation Z...but not directly. Reg Z isn't like RESPA. You would never disclose a fee that you're not charging for the loan in question. You should read the Reg Z definition of a finance charge (226.4) and its commentary. Pay close attention to 226.4(e) regarding "certain security interest charges" and (a)'s reference to "comparable cash transactions."
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The more you sweat in training, the less you bleed in battle.......