Rulings pertaining to Zakat al-Fitr
Question

Is the hadith “The fast of Ramadan will not be taken up [to Allah] until zakat al-fitr is paid” saheeh? 
If the fasting Muslim is in need and does not possess the threshold (nisab) of zakah, is he obliged to pay zakat al-fitr because of the soundness of this hadith or other saheeh shar’i evidence that is proven in the Sunnah?
Praise be to Allah.Is zakat al-fitr obligatory?
Sadaqat al-fitr is obligatory upon every Muslim who is self-supporting, if he has one sa` or more than he needs of food for himself and his family on the day and night of ‘Eid.  
The basis for that is the report narrated from Ibn ‘Umar (may Allah be pleased with him) who said: “The Messenger of Allah (peace and blessings of Allah be upon him) enjoined zakat al-fitr, a sa` of dates, or a sa` of barley, upon all the Muslims, slave and free, male and female, young and old, and he commanded that it be paid before the people went out to pray .” Agreed upon; this version was narrated by al-Bukhari. 
And Abu Sa’id al-Khudri narrated: “We used to pay zakat al-fitr when the Messenger of Allah (peace and blessings of Allah be upon him) was alive, a sa` of food or a sa` of dates or a sa` of barley or a sa` of raisins or a sa` of dried yoghurt.” Agreed upon. 
How much is zakat al-fitr?
It is acceptable to give a sa` of the local staple food such as rice etc. 
What is meant by a sa` here is the sa` of the Messenger of Allah (peace and blessings of Allah be upon him), which is four times the amount that may be held in the two hands of a man of average build. If a person does not pay zakat al-fitr, he is sinning, and he has to make it up. 
With regard to the hadith mentioned, we do not know whether it is saheeh or not. 
For more, please see these answers: 37636 , 1396 , 32751 , 145563 , 12938 , and 49632 .
And Allah knows best.