How can a principal ratify an agreement that their agent made without authority to do so?

Hypo 1: Sam owns a large investment firm. Bob is a janitor at Sam's firm. One day, while Bob was at a bar, he overheard a conversation between two potential clients. Realizing the opportunity, and despite the risk, Bob introduces himself as an associate at Sam's investment firm and represents himself as someone who can bring them on as a client. After talking, they come to an agreement. The next day, Bob tells Sam at his office what he did. Sam is at first upset that Bob would do such a thing, but then realizes how much money Bob's actions would make the company. Sam reviews the terms of the agreement, and chooses to accept them. Result: Though Sam had every right to fire Bob and refuse to be bound by the agreement he made by misrepresenting himself to the new clients, Sam's decision to ratify and accept the agreement essentially gives it a stamp of approval and officially creates a contract between Sam's firm and the new clients, even though Bob was never in a position to create such an agreement.