Firstly: There is no sin on him for exiting his ihraam and going back without completing his Hajj, because he was forced to do that. And Allaah knows best about his situation and is Most Merciful to His slaves. He will be rewarded to the extent of whatever he did of the actions of Hajj with sincerity.
Secondly: If a person stipulates when entering ihraam that if anything prevents him (from completing Hajj) he will exit ihraam at the point at which he is prevented from continuing, then he does not have to do anything, but if he did not make any such stipulation, then he must offer a hadiy to be slaughtered where he was prevented from continuing, because Allaah says (interpretation of the meaning): “But if you are prevented (from completing them), sacrifice a Hady (animal, i.e. a sheep, a cow, or a camel) such as you can afford” [al-Baqarah 2:196]. Then he should shave his head or cut his hair, and thus he will have exited ihraam.
And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote.
Standing Committee for Academic Research and Issuing Fatwas
Shaykh ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz, Shaykh ‘Abd al-Razzaaq ‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan, Shaykh ‘Abd-Allaah ibn Qa’ood.
Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah wa’l-Ifta (11/350)
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about a man who did Hajj without a permit, and he was prevented from entering Makkah. What does he have to do?
He replied: If he said when entering ihraam, “If I am prevented (from completing Hajj), then I will exit ihraam at the point at which I am prevented”, then he may exit ihraam and he does not have to do anything else. But if he did not stipulate that, then he has to slaughter a hadiy because Allaah says (interpretation of the meaning): “But if you are prevented (from completing them), sacrifice a Hady (animal, i.e. a sheep, a cow, or a camel) such as you can afford” [al-Baqarah 2:196], and he should exit ihraam (i.e., shave his head or cut his hair) at the point where he is prevented from continuing. End quote.
Majmoo’ Fataawa Ibn ‘Uthaymeen (23/433).