“I have been deceived or overreached.” Rather, an endowment is a voluntary gift which a person dedicates for the sake of Allah, Most High. It is like giving dirhams in charity without counting them: it is valid and takes effect, and it is not permissible to retract it, because he has given charity and made a voluntary donation. Therefore, the stronger view is that the endowment of a specified asset is valid even if it is unknown, since it is a purely voluntary gift: once a person executes it, it takes effect.
His statement, “from which benefit may be derived,” means: from this specified asset.
His statement, “while its substance remains,” is the important condition here. If benefit cannot be derived from it except through the destruction of its substance, then it cannot validly be endowed, because an endowment consists of retaining the principal and making its benefit available. Thus, if someone endows a sack of dates for the poor, it is not valid, because no benefit can be derived from it except by consuming it: the poor will eat it, and once they have eaten it, its substance no longer remains. Therefore, it must be a specified asset from which benefit can be derived while its substance remains.
Likewise, if he endows bread for the poor, it is not valid, because no benefit can be derived from it while its substance remains.
They exempted water from this, saying that its endowment is valid because it has been reported from the early generations (1). Thus, one may endow this water-skin for thirsty Muslims. It is said that its being reported from the early generations indicates the permissibility of similar cases, since there is no basis for singling it out as an exception.