(a) That's right. Even though this agreement is not contained in a deed, it would be legally enforceable. The reason is that, although informal, the agreement clearly involves a bargain or exchange between the parties. A's promise to build the warehouse is given in exchange for B's promise to pay him $35,000.

We can explain why the informal agreement is enforceable by saying that 'A's promise to build the warehouse is supported by consideration'. Notice that the converse is also true: B's promise to pay A is supported by A's promise to build the warehouse.

This is sometimes called the 'bargain' theory of contract.