Suppose a contract states “ten dirhams,” but two materially different kinds of dirhams circulate under that name and their monetary values diverge.
If the prevailing usage identifies neither one, the parties' obligations remain uncertain. That uncertainty over the price (thaman) can make the contract defective in the legal framework under discussion.
If one type clearly dominates usage, that custom may support a probable selection. Even so, customary preference is not identical in evidential status to an explicit identification by the speaker.
The example prepares the distinction between interpretation grounded in usage and express clarification.