All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
The endowment is a great window of charities and one of the best in terms of continuous charity, because its reward is endless and won't be stopped even if the endowment itself was consumed nor at the death of endower, since the Prophet (PBUH) said (What means): "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." [Related by At-Tirmithi and graded it as sound narration].
And this was comprehended by the righteous companions (May Allah be pleased with them) whereas most of them endowed a portion out of their assets for the benefits of Muslims, as mentioned in Sahih Al-Bukhary as Abo Talha (May Allah be pleased with him) said: "Messenger of God, God Says (What means): "You will never attain righteousness till you give freely of what you love,' and my property which I prize most is Bairaha’, so I give it as sadaqa to God most high from whom I hope for reward for the act of righteousness and the treasure relating to it;** so apply it, messenger of God, to whatever purpose God shows you.” He replied: “Bravo! That is profitable property. I have heard what you said, and I think you should apply it to your nearest relatives.” Abu Talha told God’s messenger he would do so, and divided it among his nearest relatives and his cousins on his father’s side. In addition , When `Umar (May Allah be pleased with him) got a piece of land in Khaibar, he came to the Prophet (PBUH) saying: "I have got a piece of land, better than which I have never got. So what do you advise me regarding it?" The Prophet (PBUH) said: "If you wish you can keep it as an endowment to be used for charitable purposes." So, `Umar gave the land in charity (i.e. as an endowments on the condition that the land would neither be sold nor given as a present, nor bequeathed, (and its yield) would be used for the poor, the kinsmen, the emancipation of slaves, Jihad, and for guests and travelers; and its administrator could eat in a reasonable just manner, and he also could feed his friends without intending to be wealthy by its means."
Besides, the majority of Muslim scholars are of the view that the charity set for Muslims benefit and public interest is permissible; such as the endowments set for hospitals, schools and mosques.
Therefore, the contribution in building a center for cancer patients in King Hussein Cancer Center is a continuous charity which will benefit her giver at the Day whereon neither wealth nor sons will avail but only he (will prosper) that brings to God a sound heart. And Allah Knows Best.