Don't tell her

"Uh..." you utter, forgetting the arrangement with Grace. "...magic?"

"Ha, ha," Grace responds sarcastically as she rolls her eyes. She then crosses her arms, waiting for a proper answer from you. Even though you have all the advantage here, you still feel cold sweat on the back of your neck, nervous by Grace's imposing presence. It's weird, that even after she had bent over a table and let you take her, that she's so confident and sure of herself. It makes you wonder if that was even the first time she had made such a deal.

You're actually tempted to tell her the truth, but at the last moment before you blurt out anything stupid, you change your mind and activate your mind control ring instead.

"Grace, listen to me. You'll think that I just showed you some amazing card trick that convinces you how I keep winning. However, you'll also determine that it's so complicated and difficult to pull off, that nobody else can do it, and even you can't remember exactly how to. You'll be satisfied with my answer and be good with the deal as it stands. In 5 seconds, you'll resume talking like that all just happened," you say, picking your words carefully, before you deactivate both your mind control and luck rings.

As you commanded, in 5 seconds, Grace nods like you've just told her your secret card trick.

"Interesting. Well, it's too complicated and difficult to pull off, that nobody else can do it. I can't even remember exactly how to do it, and you explained it so well. Okay. A deal is a deal. You can take the money and leave. Thank you," Grace says with a neutral expression, motioning to the door where you had entered.