Is saying "Insha'Allah" (God willing) when promising someone a visit, and then not fulfilling that promise, considered a breach of promise? What are the boundaries of the definition of a promise to which the ruling of breaching applies, and what is its expiation?

The statement "Insha'Allah" (God willing) to someone who requested a visit is not considered a promise, and no obligation arises from it, unless it is customary or the speaker's intention was that it be a promise. In that case, fulfilling it is recommended. A promise is something a Muslim commits to, and fulfilling it is recommended according to the majority of scholars, and it is among the noble virtues. It should not be broken except for a valid excuse, and it is not a binding obligation. The expiation for breaking a promise without a valid excuse is repentance and seeking forgiveness.