Praise be to Allah.
According to the scholars, whoever fails to do one of the obligatory duties of Hajj or ‘Umrah, such as shaving the head or cutting the hair, has to offer a compensatory sacrifice, to be slaughtered in Makkah and its meat distributed to the poor.
Shaykh Ibn ‘Uthaymeen was asked about a person who did ‘Umrah but did not shave his head or cut his hair because he forgot or was unaware of the ruling. What is the ruling on his ‘Umrah?
His ‘Umrah is valid even if he did not shave his head or cut his hair, because shaving the head or cutting the hair is not one of the pillars (essential parts) of ‘Umrah, rather it is one of the duties. If a person omits it by mistake because he forgot, then he should shave his head when he remembers, unless the time has passed, in which case he should offer a compensatory sacrifice in Makkah and give the meat in charity to the poor. And there is no sin on him in this case, so long as he did that out of forgetfulness or ignorance.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 22/466, 1000
He was also asked about a man who exited ihraam following ‘Umrah, after performing tawaaf and saa’i, and he did not cut his hair, then he entered ihraam for Hajj. What should he do?
It seems that he continued in the state of tamattu’, but he has to offer compensation for not shaving his head or cutting his hair, based on the well-known view among the fuqaha’ that the one who omits an obligatory duty has to offer a fidyah (ransom). If he is well off, then he has to offer a compensatory sacrifice in Makkah and distribute all its meat to the poor. If he cannot afford it, then he does not have to do anything. With regard to his Hajj, it is tamattu’ because this was his intention.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 22/468, 1004.