فِي أَرْبَعِينَ شَاةً شَاةٌ
One sheep is due in a flock of forty sheep.
The wording states the original form of the obligation. Merely mentioning a sheep does not logically prove that paying the equivalent value can never discharge the duty. Whether payment of value is valid requires a separate legal argument.
The Hanafi school allows payment of value in relevant zakat cases under its rules, while other schools differ. The text’s positive statement must be distinguished from an unstated exclusion of alternatives.