Even if you get a scale for yourself, and you find out he shorted you, is this a situation where you'd be able to force it outta him? And it's considered rude to ask a dealer to weigh it in front of you. You can end up with a black eye doing that. There are times when it's worth it to risk that though. But if your hookup isn't solid, you might wanna just ask your parents to use their sources.