لو قال: «اشتر لي جارية لتخدمني» فاشترى العمياء أو الشلاء لا يجوز
When an agent is instructed to buy a servant for service, selecting someone unable to perform the specified service does not fulfill that authority in the source example.
ولو قال: «اشتر لي جارية حتى أطأها» فاشترى أخته من الرضاع لا يكون عن الموكل
Likewise, where the stated objective is lawful intimate relations, buying a woman prohibited by a foster-sibling relationship does not satisfy the mandate.
The base words authorize a purchase, but the expressed purpose determines its operative limits. The agent cannot defend a purchase solely by appealing to the noun's general range when the purpose clearly disqualifies the item.
These are historical illustrations. The enduring interpretive point is that an agent's authority is interpreted alongside an explicitly stated objective.