وإن قال بعتكها على أنها مائة ذراع بمائة درهم كل ذراع بدرهم فوجدها ناقصة فهو بالخيار
Translation: If the seller says, “I sell it as 100 cubits for 100 dirhams, each cubit for one dirham,” and it is found deficient, the buyer has an option.
Both the aggregate price and the explicit per-cubit price are named. If the item proves to have 90 cubits instead of 100, the stated unit rate makes proportional pricing meaningful.
The phrase “he has an option” refers to the buyer. The altered extent cannot simply be imposed as the same bargain.