If everyone abandoned their commitments when circumstances changed, the concept of commitment would become nonsensical, resulting in a logical inconsistency. Therefore, according to Kant, Ben and Sarah would be bound by a categorical imperative to continue paying their mortgage if they are able. The situation is more complex, however, than Kant's moral system addresses. When applying the universal law test, we find that Ben and Sarah are equally bound to their other commitments, including their commitments to themselves and their child. The only way to find their correct moral path in the Kantian sense would be for them to intend to follow through on all of their commitments, and to do everything in their power to follow that intention, even if they are not ultimately able to. This shuts out the option of strategic default, because strategic default is an intentional dismissal of a financial obligation, something that would...

While Sarah and Ben are morally obligated to attempt to fulfill their financial commitments, should they find themselves in a situation where their livelihood is in fact endangered.
References

Atwell, John. Ends and Principles in Kant's Moral Thought. Dordrecht, Netherlands: Martinus Nijhoff Publishers, 1986.

Kant, Immanuel. Grounding for the Metaphysics of Morals, trans. James Ellington. Indianapolis: Hackett Publishing, 1981.

Pruss, Alexander. "Kantian Maxims and Lying." Georgetown University Faculty Papers. Web. http://www9.georgetown.edu/faculty/ap85/papers/KantianMaxims.html