Hi, I'm studying for an upcoming final, and doing some homework. one of your documents includes a problem i am working on, but I am having trouble understanding the answer to a question. I was wondering if you could help me understand it better so I can grasp the context.

The problem is:

Rob is selling his house in Tucson. Jim, his agent, has found a buyer, Sid. Sid signs a contract in which he agrees to buy the house for $250,000 but the document provides that "this sale is conditional on the buyer obtaining a mortgage in the amount of $200,00 at an interest rate not to exceed five (5) percent."

The document describes the problem as an "illusory promise". I need help understanding what qualifies it as an illusory promise.