What Is the Difference?

The law treats the following two situations differently: (1) those in which the offer invites acceptance either by a performance or by a promise; and (2) those in which the offer invites acceptance only by a performance and not by a promise. This creates different consequences for non-performance.

Consider (1) first. Suppose Bob's beginning performance is an acceptance of Alice's offer. Then, assuming there is no reason to excuse either party from their contractual obligations,

Bob is legally obligated to walk all the way across the bridge, and Alice is legally obligated to pay for that performance.

Bob is not legally obligated to walk all the way across the bridge, and Alice is not legally obligated to pay for that performance.