He did Hajj on its own, but he did tawaaf al-qudoom and sa‘i with the intention of ‘umrah
Question

                I did Hajj on its own (ifraad) with a group of religiously committed young men, but when I reached Makkah I said to them: What will we do now? And they said: We will do tawaaf and sa‘i. I said to them: Do you mean ‘umrah? And I did tawaaf and sa‘i with the intention of ‘umrah, because I did not know that the tawaaf and sa‘i were for Hajj, and that I did not have to do ‘umrah. Is my Hajj valid?
Praise be to Allah.Firstly:
The one who is doing ifraad (Hajj on its own) is the one who intends to do Hajj on its own: so he does not do ‘umrah before Hajj.
When he comes to Makkah, he should do tawaaf al-qudoom (the tawaaf of arrival), which is Sunnah – not obligatory – in his case. He may do sa‘i after that, and if he does sa‘i at this point, it will count as the sa‘i of Hajj; he will not need to do sa‘i again after that, according to the majority of fuqaha’.
Al-Bahooti said in Kashshaaf al-Qinaa‘ (2/411): The way in which ifraad (Hajj on its own) is done is as follows: The pilgrim enters ihram for Hajj on its own (ifraad), then when he has finished Hajj, he does the ‘umrah of Islam [obligatory ‘umrah], if he has not yet done it. End quote.
In al-Mawsoo‘ah al-Fiqhiyyah (29/121) it says:
Tawaaf al-Qudoom:
It is also called tawaaf at-tahiyyah [tawaaf of greeting], because it is prescribed for the one who comes from somewhere other than Makkah to greet the Ka‘bah. It is also called tawaaf al-liqaa’ [tawaaf of meeting] and is the first thing done when reaching the Ka‘bah.
Tawaaf al-qudoom is Sunnah for the non-resident who comes to Makkah, according to the Hanafis, Shaafa‘is and Hanbalis, as a greeting to the Ancient House. Hence it is mustahabb to start with it, and not delay it. End quote.
Secondly:
If you did tawaaf and sa‘i, and did not exit ihram, then you were still doing ifraad, and your Hajj is valid. It does not matter that you intended to do ‘umrah, because including ‘umrah with Hajj does not affect anything, according to the majority of fuqaha’.
It says in Kashshaaf al-Qinaa‘ (2/412): If someone enters ihram for Hajj, then includes ‘umrah with it, his ihram is not valid for ‘umrah, because there is no report to that effect, and there is no benefit in that, which is different to what we have discussed above, (and he does not become one who is doing qiraan [‘umrah followed by Hajj without exiting ihram in between] thereby). End quote.
If you had exited ihram – i.e., by cutting your hair or shaving your head, and putting on your regular clothes – then this was ‘umrah, and there is nothing wrong with that, because it is mustahabb for the one who is doing ifraad to cancel his Hajj and make it ‘umrah, if he has not brought the sacrificial animal with him, then to enter ihram for Hajj on the eighth day [of Dhu’l-Hijjah].
It says in Kashshaaf al-Qinaa‘ (2/415): It is Sunnah for the one who is doing qiraan or ifraad to cancel his intention for Hajj and intend that ihram to be for ‘umrah on its own, then when he has finished ‘umrah, he should exit ihram then enter ihram for Hajj, so that he will be doing tamattu‘ [‘umrah followed by Hajj, exiting ihram in between], so long as he has not brought the sacrificial animal with him, because it is soundly narrated that the Prophet (blessings and peace of Allah be upon him) instructed his companions who were doing ifraad and qiraan to exit ihram, all of them, and make it ‘umrah, except for those who had brought sacrificial animals with them. Agreed upon. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: Turning Hajj into ‘umrah so that one will be doing tamattu‘ is a confirmed Sunnah, either obligatory or highly encouraged. The correct view is that cancelling Hajj and making it ‘umrah is not obligatory; rather it is highly encouraged.
End quote from ash-Sharh al-Mumti‘ (10/315).
The evidence for cancelling Hajj is the report narrated by Muslim (1218), in the hadith of Jaabir ibn ‘Abdillah (may Allah be pleased with him) which describes the Hajj of the Messenger of Allah (blessings and peace of Allah be upon him), in which it says: Then when it was the last lap [of sa‘i] and he was at al-Marwah he said: “If I had known before what I know now, I would not have brought the sacrificial animal with me and I would have made it ‘Umrah. Whoever among you does not have a sacrificial animal with him, let him exit ihraam and make it ‘Umrah.” Suraaqah ibn Maalik ibn Ju‘sham stood up and said: O Messenger of Allah, is it just for this year or is it forever? The Messenger of Allah (blessings and peace of Allah be upon him) clasped his hands and said: “‘Umrah has been incorporated into Hajj,” twice. “No, it is for ever and ever.”
Thus it is clear that your Hajj is valid in either case, but in the former case you were doing ifraad, and in the latter case you were doing tamattu‘ and were obliged to offer the sacrifice of tamattu‘.
And Allah knows best.