What are the agent's remedies upon their principal's breach of duty?

A "lien" is a claim or legal right against assets that are typically used as collateral to satisfy a debt. In other words, it is a legal ability to claim some right to a piece of property so that it can be used as leverage to repay money that is owed to you. Thus, imagine if Sam is an art collector and Bob is his employee. Bob instructs Sam to go purchase a fancy painting and Sam will reimburse him. Bob goes and buys the painting on behalf of Sam, but Sam refuses to pay Bob. Technically, Bob purchased the painting for Sam as an agent of Sam, so Sam has a legal right to the painting. However, because Sam has breached his agreement with Bob, Bob is allowed to get a lien on the painting.

What does this mean? It means Bob doesn't have to surrender the painting to Sam and can, instead, hold the painting hostage until Sam reimburses him for the cost. It's a legal way for Bob to avoid being accused of violating his duties to Sam as his agent.