ونفلًا في حق الاعتقاد، فلا يلزمنا الاعتقاد به جزمًا
Regarding assent, wajib does not require the same categorical conviction as a definitively established ruling.
Calling it “like nafl with respect to assent” is a technical comparison about the degree of certainty. It does not cancel its binding practical force. Scholarly disagreement about the basis of a wajib may therefore be legitimate.
Fard entails both performance and the acknowledgment appropriate to definitive proof. Wajib likewise requires action, but the non-definitive nature of its evidence means that the same level of certainty in assent is not imposed.