All perfect praise be to Allah the Lord of the Worlds. May Allah`s peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Continuous charity is considered endowment based on the Hadith of Abu Hurairah (May Allah be pleased with him) who reported:
The Messenger of Allah (PBUH) said: "When a man dies, his deeds come to an end, except for three: A continuous charity, knowledge by which people derive benefit, pious son who prays for him." [Muslim].
Commenting on this Hadith, An-Nawawi (May Allah be pleased with him) said: "Continuous charity is considered endowment" {Shareh Muslim, vol.1/pp..85}. Al-Shirbini (May Allah have mercy on him) said: "In Islamic terms, waqf refers to a religious endowment i.e. a voluntary and irrevocable dedication of one's wealth or a portion of it - in cash or kind (such as a house or a garden), and its disbursement for Shariah compliant projects (such as mosques or religious schools...). Scholars have supported the view of al-Rafe`I who considered continuous charity endowment since other charities aren`t as such. Rather, the person receiving the charities immediately owns the dedicated items and their benefits. As for bequeathing the benefits, this is rare. Therefore, it is more accurate to consider continuous charity, mentioned in the above Hadith, endowment." {Moghni al-Mohtaaj, vol.3/pp.522-523}.
In conclusion, whoever donates money with the intention of authorizing the above Society to purchase a plot of land to build its headquarters on to be an endowment for Allah`s sake, then it is a continuous charity for which he will be rewarded in life and after death. And Allah The Almighty Knows Best.