Is it valid to suspend the termination of an endowment upon a condition?

Primary text

If the termination of the endowment is suspended upon a condition, such as saying, 'My house is an endowment until one year,' or 'until the pilgrims return,' it is invalid according to one opinion because it contradicts the essential nature of endowment, which implies perpetuity. According to another opinion, it is valid because it resembles an endowment with a finite duration, thus being treated as such.