Rasulullâh Sallallâhu ‘alayhi wasallam said:
“The fast remains suspended between Heaven and Earth until the fitrah is paid.”
Sadaqatul fitr [fitra] is compulsory charity which becomes payable on the occasion of Eid-ul-Fitr.
Fitr means to break fast or to refrain from fasting. Hence the Eid after the fasts of Ramadhân is called Eid-ul-Fitr, as it is the day of rejoicing after the completion of fasting. On this occasion of happiness, as a sign of gratitude one has to give a specific amount in aims, which is called Sad’qatui-Fitr. To purify and obtain complete blessings for the fasts of Ramadhân Mubârak one has to give Sadaqatul-Fitr. Thus it is wâjib to give Sadaqatul-Fitr to purify one’s fast. The true object in giving Sadqatul-fitr on this happy, occasion is also to assist the poor and needy, so that, they may rejoice with the more fortunate.
Ibn Abbas (ra) reported that the Prophet (saw) made the Sadkat-ul-Fitr obligatory for the purpose of: purifying our fasting from vain talk and shameful mistakes, to make arrangements for the poor and the needy for food and clothing (for the festival of Eid). (Hadith: Abu Dawud, Ibn Maja)
Sadaqatul Fitr (or fitrah) is wâjib [compulsory] upon all Muslims -male, female and children fitrah should preferably be paid before the Eid Salâh.
It is not permissible to delay the payment of fitrah later than the Day of Eid.
What amount Should be paid
The amount of Sadqatul Fitr is 1.633kg of wheat or 3.266kg of barley or the equivalent in cash. Please consult your local Islamic Scholars[Ulema] in your area for exact amount.It is highly recommended that this amount is paid in advance so that the contributions could be used for the benefit of the poor on Eid day.
source : eislaminfo.blogspot.co.uk