ولا يجوز السلم بمكيال رجل بعينه ولا بذراع رجل بعينه
Translation: Salam is not valid using a particular person’s undefined vessel or cubit as the measuring standard.
If a contract specifies one person's personal vessel without anyone knowing its exact capacity, the quantity owed is unstable. If the vessel disappears or its size is disputed, the buyer's right may become impossible to establish.
The same reasoning applies to a private cubit measure of unknown length. Fath al-Qadir explains that a known, reproducible capacity avoids the problem; the prohibited feature is an unknown standard, not the mere naming of a person's vessel.