When does Khums become Wajib?

Khums should be divided into two parts. One part is for the Sādāts which should be given to a sayyid who is poor, or orphan or who has become penniless during journey. The other portion of Khums is for Imam az-Zaman (a.s.)

Khums is obligatory on the following seven things:
(i) Profit or gain from earning.
(ii) Minerals.
(iii) Treasure-trove
(iv) Amalgamation of Halāl wealth with Harām.
(v) Gems obtained from diving into sea.
(vi) Spoils of war.
(vii) A land which a Zimmi Kafir purchases from a Muslim.
The detailed laws for each of the above have been discussed in Tauzihul Masael. Khums should be divided into two parts. One part is for the Sādāts which should be given to a sayyid who is poor, or orphan or who has become penniless during journey. The other portion of Khums is for Imam az-Zaman (a.s.) which should be paid to his qualified representative during Ghaibat or it should be spent in the way permitted by the representative who is fully qualified.
The book Kalematul Tayyabah has forty incidents that illustrate the benefits of helping the Sadāt and the merits of being kind to them. We shall be content to narrate one of those incidents here.
The incident has been recorded in books like Arbaīn, Muntakhabul Dīn, Fazael al-Shazān, Tohfatul Ahzār and Wasīlatul Māl. It is narrated through authentic chain of narrators that Ibrahīm bin Mehran said:
In Kufa I had a good-natured neighbour by the name of Abu Ja’far. Whenever a Sayyid came to him asking for something he would give it to him. If he paid for it Abu Ja’far accepted the payment, if he didn’t, however, Abu Ja’far instructed his assistants to record it in the ledger of debts under the name of ‘Ali Ibn Abi Talib (a.s.).
This continued for quite some time and when his financial conditions worsened and he became poverty stricken he took out the ledgers and tried to recover his debts. If the debtors were alive he sent his servants for recovery of debt but if the person was no more or had nothing to repay, Abu Ja’far would strike out the name from his register.
One day he was sitting near the entrance of his house poring over the ledgers and a Nasibi (one who is inimical to ‘Ali (a.s.) passed by and taunted Abu Ja’far.
“How about your greatest debtor – ‘Ali? Did he repay your debts?” Abu Ja’far was hurt at this taunt and the same night he dreamt that the Holy Prophet (S) was sitting with Imam Hasan and Imam Husain (a.s.). The Messenger of Allah (S) asked, “Where is your father?” Amir ul-Mu’minīn ‘Ali (a.s.) replied from behind. The Prophet (S) asked him why he had not restored the debt of Abu Ja’far.
‘Ali (a.s.) said, “I have brought this for repayment.” So saying he took out a white woollen bag and gave it to Abu Ja’far. The Holy Prophet (S) told him to take it and whenever their descendants came to ask him for something he should not spurn them and added that after this there shall be no problem for him.
Abu Ja’far awoke and saw that he was holding the same bag that ‘Ali (a.s.) had given him. He shook his sleeping wife and told her to put on the light. When the bag was opened it was found to contain one thousand Ashrafis (Gold Coins). The wife said, “O Allah’s slave! Fear Allah, I hope the difficult times have not compelled you to obtain this money by cheating other traders.” “Certainly not!”, said Abu Ja’far and narrated the whole incident. Then he got out his ledger and calculated the total amount in ‘Ali Ibn ‘Ali Talib’s account. He found it to be exactly 1000 Ashrafis. Neither less nor more.