Rav was appointed by the Raish Galusa to be the angarmus (to supervise the measures). Rav would give lashes to anyone who used false measures but not to a person who sold for above the market price.

The Raish Galusa imprisoned Rav (for not punishing when a person overcharged). Rav Karna went to visit Rav.

Rav to Rav Karna: An angarmus's job is only to supervise measures but not overcharging.

Rav Karna to Rav: Didn't you teach us that he must also supervise overcharging?

Rav to Rav Karna: Go out and expound to them that an angarmus's job is to supervise measures but not overcharging.

Rav Karna went out and told the students, "The one who expounded this law (i.e. Rav) is like a jar of fine dates (i.e. he taught correctly) and you imprison him?''