If a person does not attain the required capability (Isteta’at), then hajj is not wajib on him/her. If such a person performs the hajj without the required capability, then his hajj is OK, but he has to perform the hajj again in future if and when he attains the required capability (isteta’at).
It is not wajib on a person to try to achieve the required capability in order to perform hajj. This is not the case for other acts of worship such as wudu for wajib salat, for which it is wajib to try to get the water for performing wudu.
The wajib hajj is of two types depending upon how it becomes wajib :
1. hajje Isteta’ei (wajib when attaining required Isteta’at automatically)
2. hajje Bazli (wajib when someone gifts sufficient money to someone for hajj)
When someone gives you sufficient money as a gift for performing the hajj, then it is wajib for you to accept the gift and go for hajj. For example, if husband is willing to pay for his wife or for his baligh (mature) child for hajj, then it is wajib for the wife or that child to go for hajj. If they go to hajj with this gifted money, then they have performed their wajib hajj and need not go again even if they get their own sufficient money in future.
If someone is giving sufficient money as a gift to someone without putting a condition to go for hajj, then it is not wajib for the receiver to accept the money. But if he accepts and the money is sufficient and other conditions of isteta’at exist, then he has to go for hajj.
If a person, on whom hajj is not wajib (e.g. na-baligh person (immature), or when the passage is not safe), performs the hajj somehow, then he will get the sawab of that hajj, but he will have to go to do his wajib hajj in future when hajj becomes wajib on him.