Isar means preferring others to oneself and includes giving to others what one needs himself. Sacrificing one’s life, properties, position, etc, on the path of God is a kind of Isar.
It is among the most beautiful signs of humanity. The Holy Quran has pointed to examples of Isar and praised it. For example, the Ansar (Helpers) were the people of Medina who, despite their own need, gave their belongings and houses to those coming from Mecca accompanying the Holy Prophet (PBUH).
“And those before them who had made their dwelling in the abode (the City of Medina), and because of their belief love those who have emigrated to them; they do not find any (envy) in their chests for what they have been given and prefer them above themselves, even though they themselves have a need. Whosoever is saved from the greed of his own soul, they are the ones who win.” (Surah Al-Hashr, Verse 9)
The most important thing in Isar is purity of intention and avoiding hypocrisy or making others feel obligated.
According to the Holy Quran: “Believers, do not make your charities fruitless by reproachfully reminding the recipient of your favor or making them feel insulted, like the one who spends his property to show off and who has no faith in God or belief in the Day of Judgment. The example of his deed is as though some soil has gathered on a rock and after a rain fall it turns hard and barren. Such people can not benefit from what they have earned. God does not guide the unbelievers.” (Surah Al-Baqarah, Verse 264)