It is prescribed for the one who wants to enter ihram for Hajj or ‘Umrah to stipulate a condition [about exiting ihram] when entering ihram, if he is afraid that some obstacle may prevent him from completing Hajj and ‘Umrah. So he should say: “If something prevents me (from continuing), I will exit ihram from the point where I am prevented”, because of the report narrated by al-Bukhaari (5089) and Muslim (1207), according to which the Messenger of Allah (blessings and peace of Allah be upon him) said to Dubaa‘ah bint az-Zubayr when she wanted to do Hajj but she was sick: “Go for Hajj but stipulate a condition and say: O Allah, I will exit ihram from the point where You have prevented me (from continuing, if some problem should arise).” 

The benefit that the pilgrim in ihram gains from that is that if something happens to him that prevents him from completing the rituals, such as sickness or an accident, or he is prevented from entering Makkah for some reason, then he may exit his ihram and he does not have to do anything – he does not have to pay any ransom (fidyah) or offer a sacrifice (hadiy) or shave his head. 

Were it not for this stipulation, he would be muhsar, i.e., prevented from completing the rituals, and he would have to slaughter a sacrificial animal and shave his head, as the Prophet (blessings and peace of Allah be upon him) did at al-Hudaybiyah, when the mushrikeen prevented him from entering Makkah. So the Prophet (blessings and peace of Allah be upon him) slaughtered his sacrifice and shaved his head, and he instructed the Sahaabah to do likewise and said to them: “Get up and offer your sacrifices, then shave your heads.” Narrated by al-Bukhaari, 2734. 

Allah, may He be exalted, says (interpretation of the meaning):

“And perform properly (i.e. all the ceremonies according to the ways of Prophet Muhammad (blessings and peace of Allah be upon him)), the Hajj and Umrah (i.e. the pilgrimage to Makkah) for Allah. But if you are prevented (from completing them), sacrifice a Hady (animal, i.e. a sheep, a cow, or a camel, etc.) such as you can afford, and do not shave your heads until the Hady reaches the place of sacrifice”

[al-Baqarah 2:196]. 

Shaykh Ibn Baaz (may Allah have mercy on him) said: 

The benefit of this condition is that if something happens to the pilgrim in ihram that prevents him from completing his rituals, such as illness or having his way blocked by an enemy, it is permissible for him to exit ihram and he does not have to do anything. End quote. 

Majmoo‘ Fataawa Ibn Baaz, 17/50 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

As for the benefit of stipulating the condition, the benefit is that if something happens to the person that prevents him from completing his rituals, he exits ihram and does not have to pay any ransom (fidyah) or make up his (Hajj or ‘Umrah).

End quote. 

Majmoo‘ Fataawa Ibn ‘Uthaymeen, 22/28.