It is an amount that person has fixed for himself which he will spend on the needy and his relatives. It can be every day, every week or every month. Thus, the Almighty says in Qur’an:
“And those in whose wealth there is a fixed portion. For him who begs and for him who is denied (good).” (Surah Ma’ārij 70: 24-25)
It is narrated from Imam Mūsa Ibn Ja’far (a.s.) that Bani Israel was a righteous man who had a kind hearted wife. One night this man dreamt that he was told that the Almighty had fixed a particular life span for him. Half of it was to be in prosperity and other half in difficulties. He had the choice of whichever life he desired first and whichever condition he desires to have in the later life. The man requested for some time as he wanted to consult his wife as he had always sought her counsel in important matters. Next day he related the dream to his wife who told him to ask for a prosperous life first. She said, “And hurry up to obtain it. May be Allah (S.w.T.) desires to grant us with His bounties.”
The next night when he was asked in his dream he said that he wished for prosperity in the earlier part of life. He was told that his wish shall be fulfilled. Thus he began to receive all types of bounties and comforts. His wealth and property increased. Then his wife said, “O slave of Allah! Now you help your relatives and other deprived people. Do good to them. Give that particular thing to that neighbour and something to that friend of yours.”
This man followed the advice of his wife and was not stingy in giving his wealth in charity. In this way half the life passed. Again he dreamt of the same person and he was saying, “You were not stingy in charity, therefore the Almighty had decided to grant you prosperity in the later half of your life also.”