Greta couldn't help laughing at Z's obvious frustration.
"Okay, okay! That's a lot of cards. I hope I didn't cause any serious injury to your Slytherin sneakiness credentials by making you admit to all that. Anyway, I'm honestly not trying to blackmail you. Or bribe you, whatever." Greta thought about that for a moment. "Well, mostly not trying to bribe you."
She picked out another broom and checked it over just like she had the other one: also fine, needed some work on the twigs to prevent drag.
"The truth is, I could use some help going over the Quidditch equipment before term starts, and if you've been polishing the brooms you borrowed anyway, you might as well do the same with the school equipment. And then of course it would make sense for me to write you a thank you note for your responsible care and handling of the school brooms. If you felt like showing that to your parents, it's no business of mine."