Is the agent's sale of the principal's property without his consent, and at a price unsatisfactory to him, considered a breach of trust, even if based on a fatwa?

The agent is not permitted to exceed the scope of his agency. If he violates the terms set by his principal, or sells for less than the market price or the stipulated price, then he is an transgressor and liable for damages. If the agent is dismissed and the agency is entrusted to another, then he is thereby relieved of his duties.