Tell her

Without much choice you swallow your pride and tell her about your mission.

"I am going to save the princess, take out the witch and I thought I could work together with you." You say with clenched teeth as her hand keeps tightly squeezing your nuts.

The thief takes a moment to consider her options.
"Hmmmm, you are lucky I like you. You were not completely terrible in bed and you are a fine warrior. Together we can take that witch out and get some money." She says thoughtfully.

We split the reward. You reply.

She gives another hard squeeze in your nuts and you cry out in agony.

"I want 65% of the reward loverboy. I am a classy lady not some cheap bitch working for some pocket change." She says sternly.

"Right, right! 65%. I give you my word of honor that I will keep this promise." You cry out.

The thief lets go off you. She knows that now you have given your word you are bound to keep the agreement.
She gives you a sultry smile.

"So... what is your plan on confronting that witch?"