The text also limits analogy in substitution: where nothing resembles a duty in either outward form or operative meaning, qada cannot simply be invented on the basis of likeness. In this context the book invokes the maxim «المنافع لا تضمن بالإتلاف» concerning lost benefits.
Conversely, a legally specified substitute may exist even without the same outward form, but it requires its own legal basis. The book’s example of fidya is not a general licence to replace any missed worship with an arbitrary act.
Thus “any later act can replace any missed obligation” is not a valid usuli rule.