1: If a person earns by means of trade, industry or any other ways of earning, like, if he earns some money by offering prayers and fasting on behalf of a dead person, and if it exceeds the annual expenses for maintaining himself and his family, he should pay Khums (i.e. 1/5) from the surplus, in accordance with the rules which will be explained later.
2: If a person acquires wealth without having worked for it , like, if someone gives him a gift, and that wealth exceeds his own annual expenses, he should pay Khums from the excess.
3: There is no Khums liability on Mahr which a wife receives, nor on the property, which a husband gets in exchange of divorcing his wife by way of Khula , and the same rule applies to the property which one inherits according to the genuine laws of inheritance.
If a Shia Muslim inherits from a source which is not accepted in our Fiqh, like inheriting from a distant relative despite his heirs being present (Ta'seeb), it will be considered a gain, and Khums will have to paid from it.
Similarly, if a person inherits from an unexpected source, neither from his father nor from his son, then as an obligatory precaution, he will pay Khums from that inheritance if it exceeds his annual expenses.
4: If a person inherits some property and knows that the person from whom he has inherited did not pay Khums from it, he (the heir) should pay its Khums. And if that property is itself not liable for Khums, but the heir knows that the person from who he has inherited, owed some Khums, he should pay it from the deceased's estate. But in both the cases, if the person from whom he inherits did not believe in Khums, or never paid it, then it is not necessary for the heir to pay off the Khums owed by the dead.
5: If a person saves from the annual expenses because of economising and frugality, he should pay the Khums.
6: If the expenses of a person are borne by somebody else, that person should pay Khums on his entire earning.
7: If a person purchases a commodity with the money on which the Khums has not been paid, that is, if he says to the Shia Ithna Asheri seller: "I am purchasing this commodity with this money," the transaction will be in order in respect of the entire property, and Khums will apply to the commodity which he has purchased with that money. And no permission and acknowledgement of a Mujtahid will be necessary.
8: If a person purchases a commodity, and after the transaction, pays its price from the money from which Khums has not been paid by him, the transaction will be in order, but he will be indebted to those who deserve to receive Khums, for the sum he has paid to the seller.
9: If a Shia Ithna Asheri person purchases something on which Khums has not been paid, the Khums will be the liability of the seller, and the buyer is not responsible for anything.
10: If a person gives a gift to a Shia Ithna Asheri, from which Khums has not been paid, one fifth of it is the liability of the donor himself, and one who gets the gift is not required to pay anything.
When khoms is wajib on earning
1: It is obligatory on the merchants, the earners, the artisans, and others like them that when a year passes since they started earning, they should pay Khums from whatever is in excess of their expenses for one year.
And if a person who is not earning, makes an unexpected gain, he should pay Khums after a year has passed since he gained, on the savings which exceeds his expenditure for that year.
2: A person can pay Khums as and when he earns a profit during a year, and it is also permissible to delay payment of Khums till the end of the year, but if he feels that there will be no need to use that many until the end of year so in this situation on obligatory precaution he has to pay Khums straightaway, And there is no objection if one adopts the solar year for the payment of Khums.
3: If a person cannot prepare all the dowry for his daughter at the time of her marriage, and has to do so over a few years, and if it is deemed unbecoming for him not to give away any dowry, Khums will not be liable on what he purchases during the year, provided it is within his means. But if he exceeds his means, or spends the profit of one year to buy the dowry in the following year, he will pay its Khums.
4: Whatever a person spends for his journey to Hajj and other Ziyarats (pilgrimages) is reckoned to be part of his expenditure of the year in which he spends it, and if his journey extends till part of the next year, he should pay Khums on what he spends during the second year.
5: If a person purchases provision for his use during the year, with the profit made by him, and at the end of the year a part of it remains unused, he should pay Khums on it. And if he wants to pay its value, which may have increased since he brought the provision, he should calculate the price prevailing at the end of the year.
These laws are according to Aayatullah Sistani's Fataawa.