Reference for Ketubot 147:7

תנאה דאפשר לקיומיה ע"י שליח כי התם הוי תנאיה תנאה דלא אפשר לקיומיה ע"י שליח כי התם לא הוי תנאה

A condition that may be carried out through an agent, as was the case there, is regarded as a valid condition; but one which cannot be carried out through an agent, as was the case there, is not regarded as a valid condition.

Jastrow