• The inquirer owns a piece of desert land and enough money to buy two apartments for his two sons.
• He wants to perform hajj along with his wife, but the expenses average around 40% of the total sum of money saved for the two apartments.
Should he make hajj or keep the money to buy the apartments and help his sons out to settle down in preparation for their marriages?

Hajj is the 5th pillar of Islam

Hajj is an integral pillar of Islam; Allah the Almighty made it incumbent upon those of His worshippers who are capable of undertaking it. He says:
Pilgrimage to the House is a duty owed to Allah by people who are able to undertake it. Those who reject this [should know that] Allah has no need of anyone. [Qur`an 3:97]

Hasten to answer the call of Allah
Abu Umama (may Allah be pleased with him) narrated that the Prophet [pbuh] said: "Whoever dies without performing hajj while not being prevented from it by illness, a manifest necessity or oppression from an unjust ruler, then let him die as a Jew or a Christian" [Al-Baihaqi related it in his Sunan].
And in Abu Hurayra's narration the Prophet [pbuh] said: "Make hajj while you have the means" [Al-Baihaqi related it in his Sunan].
It is better for a Muslim to hasten to perform the obligation of hajj if he has the means and is physically able to undertake it since he does not know what the future holds.

The Ruling
The inquirer is to hasten to perform hajj if he is the sole owner of the money. He is not to please his sons and deprive himself from this obligatory act of worship, especially since his sons are adults and hold good paying jobs. Allah says:
And in the heaven is your provision and whatever you are promised. [Qur`an 51:22]
There is not a creature that moves on earth whose provision is not His concern. He knows where it lives and its [final] resting place: it is all [there] in a clear record. [Qur`an 11:6]
We must rely on Allah alone. The Prophet [pbuh] said: "If you were to rely on Allah as He should be relied upon, He would provide for you as He provides for birds. They go out in the morning hungry and return in the evening bellyful" [Al-Tirmidhi].

On Allah our Sustainer we rely and Allah Almighty knows best.