You don't want to weigh a baggy infront of him, yet you're rude enough (in the dealers eyes) To weigh it in front of him, showing distrust (and therefore a complete lack of respect)

Unless of course he's offered you to weigh it but otherwise don't complain if he tells you to fuck off.
Frankly he has every right to unless he's clearly taking the piss and the bag looks SEVERLY undercut.

Get your scales, buy from him, weigh it at home (and the baggy since they'll have different weights depending on size) oh - And maybe go back to your school and demand an education.

Jon