In Hanafi terminology, farḍ is established by definitive evidence and wājib by non-definitive evidence. Both may bind conduct, but the epistemic status of their proof and the consequences of denial are not identical.
A concession raises another issue. If the law provides relief for a recognized excuse, the original norm has not thereby vanished for everyone, and the concession has not become unconditionally available to all.
For example, concessions related to travel depend on a legally qualifying journey, the relevant worship, and its conditions. The rule is not “hardship eliminates every obligation”; the concession’s scope must be determined from legal evidence.