Is it permissible to borrow a share of communal water rights outside of one's scheduled turn?

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It is permissible for a person to borrow a turn of water from another co-owner when that person needs water outside of their own designated turn, provided they intend to repay the equivalent amount on their scheduled turn. However, Imam Ahmad disliked this if the amount was not specifically defined (i.e., measured), because the inability to return an exact equivalent is problematic.