You wouldn't really use the same scale to weigh Fentanyl and weed. Fentanyl is dosed in micro grams. $10k of fentanyl requires a much more precise scale than $10k of weed. Typically such a precise scale cannot handle the weight of $10k of weed.

You'd have to have a dealer who sells very large amounts of Fentanyl and very small amounts of weed.