A man who says he is in ihram for a Hajj: when does Hajj become obligatory on him?

Ibrahim said: If someone swears, "The day I do such and such, I am in ihram for Hajj," then when he breaks the oath he becomes a muhrim (in the state of ihram). And if he swears, "If I do such and such, I am in ihram for Hajj," and the month of Shawwal has entered, he will be counted as a muhrim.

Al-Sha'bi said: If a person says, "If I do such and such, I am in ihram for Hajj," then he will perform Hajj along with the people (after breaking the oath).

Something similar is narrated from al-Sha'bi.

Al-Hakam said: If a person says, "The day I do such and such, on that day I am in ihram for Hajj," and then he breaks the oath, he will be counted as being in ihram for Hajj on that day. And if he says, "If I do not do such and such, I will be in ihram for Hajj," then if, after breaking the oath, he performs Hajj along with the people, it will suffice for him.