Giving sadaqa al-fitr becomes wajib upon every Muslim who has property or money as much as the amount of nisab in addition to his or her indispensable belongings and debts. If he or she does not have property or money as much as the amount of nisab, it does not become wajib upon him or her to give sadaqa al-fitr.

It is declared in hadith-i sharifs:

(The fast of Ramadan remains suspended between the sky and the earth. It goes up when sadaqa al-fitr is given.)[Abu Hafs]

(Sadaqa al-fitr purifies a fasting person from the sins that he incurred because of his obnoxious utterances.)[Bayhaqi]

(Sadaqa al-fitr is a means of purification for your rich ones. If your poor ones also give it, Allahu ta'ala grants them a lot more than what they have given.) [Abu Dawud]

According to the other madhahib, giving sadaqa al-fitr is fard (obligatory) upon a person who has a day's food. A hadith-i sharif says, "Everyone, young or old, rich or poor, must give sadaqa al-fitr" (Abu Dawud).

Any person who does not meet the conditions specified by Islam to be considered rich can receive sadaqa al-fitr and zakat (obligatory alms). Giving sadaqa al-fitrbecomes wajib upon every Muslim who has property or money as much as the amount of nisab in addition to his or her indispensable belongings and debts. It is haram for such a person to receive sadaqa al-fitr or zakat. The property that is to be included into the calculation for sadaqa al-fitr does not necessarily have to be intended for trade, nor does one have to have had it for one year.

Richness that is known by the public is different from richness defined by Islam. One who does have property or money as much as the amount of nisab is considered poor by Islam. If one who does not own a house or who lives in a rented house has money or gold or commercial property as much as the amount of nisab, one is considered rich by Islam. Such a person must pay zakat and is not permitted to receive zakat.

Zakat is not paid for property that is not for trade, but income from such kinds of property is included into the calculation of nisab.

Those who do not have money or property as much as the amount of nisab are considered poor. Such people can receive zakat. If those with minimum salaries have money or property as much as the amount of nisab after they deduct their debts, they are considered rich and must pay sadaqa al-fitr. [The amount of nisab is 96 grams of gold, or its money or commercial property equivalent.]

Sadaqa al-fitr is given in the holy month of Ramadan. Though it is also permissible to give it before Ramadan or after the Eid, there will be more blessings if it is given before the Eid prayer. It cannot be given before Ramadan in the Shafi'i Madhhab, and it cannot be postponed until after the Eid, either. One who cannot fast because of a good excuse, such as illness, must pay the sadaqa al-fitr if one is rich.

One cannot pay sadaqa al-fitr to one's parents, to any of one's grandfathers or grandmothers, to one's own children or grandchildren, to one's wife, or to a disbeliever. It is permissible to pay sadaqa al-fitr to one's poor daughter-in-law, son-in-law, mother-in-law, father-in-law, brother-in-law, or stepchild. It is a lot more rewarding to give sadaqa al-fitr to such relatives as one's paternal aunt, paternal uncle, maternal aunt, or maternal uncle. According to Imamayn (Imam-i Muhammad and Imam-i Abu Yusuf), a woman can give sadaqa al-fitr to her indebted and poor husband. (Mawkufat)

The amount of sadaqa al-fitr to be paid does not vary from year to year. For sadaqa al-fitr, half a sa' of wheat or flour is given. Or one sa' of barley or dates or raisins is given. Half a sa' is a measure that equals 1750 grams circumspectly. One sa' equals 3500 grams. These amounts do not ever change until Doomsday. For sadaqa al-fitr, wheat or flour or barley or dates or raisins are given. Or it is also permissible to give their gold or silver equivalents. If it is difficult also to give wheat or flour or the other foodstuffs, one may give bread or corns of equal value.

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to understand the answer better, please look at meanings of the Islamic words:

fitra: alms (2 kg of wheat or silver of equal value) given after every Ramadan, the ninth month of Muslim calendar.

wajib: (act, thing) never omitted by the Prophet, so almost as compulsory as fard and not to be omitted. Wajib al-wujud: Being whose existence is indispensable and nonexistence is impossible.

nisab: minimum quantity of specified wealth which makes one liable to do some certain duties.

madhhab: all of what a profound 'alim of (especially) Fiqh (usually one of the four-Hanafi, Shafi'i, Maliki, Hanbali) or iman (one of the two, namely Ash-ari, Maturidi) communicated.