Is a lease contract valid if the starting time for a journey is unspecified?

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A lease contract (Ijtira') for something like a tent (*fustat*) to Mecca is invalid if the starting time of the departure is not specified. This ruling is held by Abu Thawr and is analogous to the position of Al-Shafi'i. The basis for invalidity is that the duration has an undetermined commencement, which is impermissible, similar to leasing a house from the time the pilgrims depart until the end of the year. Arguments suggesting validity are considered contrary to textual evidence.