«لفلان علي مائة ودرهم»
“I owe so-and-so one hundred and a dirham.”
The initial “one hundred” gives a number but does not identify its unit. “And a dirham” supplies a named monetary unit capable of being owed. Under this rule, the conjunction identifies the earlier hundred as dirhams.
Result: 100 dirhams plus 1 dirham equals 101 dirhams. The key intellectual step is not addition but determining the unit of the previously unspecified hundred.
Ignoring the link between the later named unit and the earlier number would leave the admission indeterminate. Here the explicit item following the conjunction supplies the needed specification.