One of the instances of khums that include most people in the society is giving khums on that which exceeds a person and his families yearly expenses (benefit/profit from working).Islam respects a person working and earning, and therefore allows the necessity of one's needs over that of paying the khums.
Therefore, one who can secure his (financial) needs from his earnings, and if after securing his (financial) needs, nothing remains at the end of the year, khums will not be obligatory on him.
But, if after working out all the normal expenses for his life (and the maintainence of his family) according to his (social) status and needs, and without being extravagent or miserly, if something remains at the end of the year, 1/5 (or 20%) of that which remains must go towards the specified expense (of khums), and 4/5 (80%) of the amount that is left can be put into one's savings.
ISSUE 381: Until khums has been paid on one's property, one is not able to spend from that property, meaning, one is not allowed to eat that food on which khums has not been taken out on, and similarly one is not allowed to spend that money on which khums has not been taken out on, to buy things.
ISSUE 382: If at the end of the year, the provisions or foods such as rice, oil, or tea, that had been purchased from one's wealth for use during the year, remain in excess (any extra remains), then khums must be paid on them (the excess).
ISSUE 383: If a non ـ baligh child has some money, and from that money receives a profit, the paying of the khums becomes wajib on the child's guardian, and if he does not pay the khums for this property, then once the child becomes baligh, he must pay the khums.