Responsa for Bava Kamma 208:1

Q. A erected a building on premises owned by a widow in spite of the latter's protests and warnings not to do so. She now demands that A remove his building and that he pay her rent for the time the building stood on her premises. A claims that he rented the space for his building, for ten years (at an annual rental of ten denarii, one-half pound^*The rental price given above is mentioned in the answer only, and may represent. therefore, a mere arbitrary sum assumed by R. Meir instead of the actual rental.), from the widow's son and son-in-law; that the latter two informed the widow of the transaction, and that the widow sent them her written consent thereto. The widow denies that she ever gave her consent to lease her courtyard to A.
A. Even A's producing a written consent from the widow would be of no avail unless he can also prove through witnesses that the widow instructed and ordered the writing of the instrument. Should he not be able to do so, he will have to remove the building from the widow's premises and pay her rent for the time his building stood thereon. Should A produce such witnesses, his contract with the widow's son and son-in-law will be non-voidable even though A did not yet pay the rent for the full ten years. Although the widow did not personally instruct her son and son-in-law to rent her premises to A, but sent such instructions in writing — such manner of sending instructions not being acceptable in talmudic law — nevertheless the son and son-in-law became the widow's agents, since it is the usual custom of merchants to accept as valid written instruments, and since the custom of merchants prevails in business transactions.
This Resp. is addressed to R. Eliezer ha-Kohen, and R. Eliezer.
SOURCES: Pr. 698.