Ruling 2278
A person must not become an agent to perform a task that he is not capable of performing or is legally (sharʿan) prohibited from performing. For example, a person who is in the state of iḥrām[1] for hajj and is therefore not permitted to say the formula for a marriage contract cannot become an agent for someone to say the formula for him.
-Ayatullah Sistani, Practical Laws of Islam, Agency ((Wikalah)
1
Iḥrām here refers to the state of ritual consecration of pilgrims during hajj and ʿumrah.