Tell her the truth

"I need your underwear. I made a bet, and it's nearly the deadline, so if you could do this for me, I promise I'll pay you back."

"A bet for my panties?" Chloe asks inquisitively. "Or just any girl's?"

"Specifically, I need a jock's."

"Oh, interesting. So what do I get out of this?"

"A favour?"

"Alright, whatever," Chloe says, leaning in and grabbing the cigarette from you.

Chloe takes a drag, before handing it over to you.

"You get us a joint or two sometime, maybe we hang out and you return the favor," Chloe says, sliding her fingers into her short's waistband.

Chloe lowers her shorts, revealing a pair of light green cotton underwear, now stained with sweat. She lowers them down, revealing a fuzzy blonde landing strip leading down to her pretty pink pussy, the outer labia peeking just out of the slit like butterfly wings. She offers you the underwear, which you take gratefully.

"Thanks. I appreciate it."

"Don't mention it," Chloe grins, seeming unembarrassed of the whole affair.

"Guess I'm going commando until I can get back to the dorm, shower and change."

You look at the underwear, seeing the damp sweat spots on the front and back. It must smell intoxicating.