Final answer:
Joseph may sue Ralph for breach of contract under promissory estoppel because he incurred expenses based on Ralph's promise, which Ralph later prevented from being fulfilled by building a fence.
Step-by-step explanation:
The student's question pertains to a scenario where Ralph has promised to pay Joseph to paint his house, but then subsequently prevents Joseph from fulfilling the contract by erecting a fence. In this case, Joseph may sue Ralph for breach of contract under the legal doctrine of promissory estoppel, which protects a party who reasonably relied on the promisor's pledge when that reliance results in a substantial economic loss. Since Joseph already incurred expenses by purchasing paint and supplies in reliance on Ralph's promise, he can argue that Ralph's actions have caused him a financial loss.