Amirul Momineen Ali (a.s.) says in Nahaj-ul-Balagah: Zunbun…Maghfoor meaning: Sins are of three kinds: The one which will be forgiven, two; the committer of which can hope for pardon and three; the sin which will never be forgiven.
The forgivable sin will be that which might have been paid for in the world. For example, the offender might have been punished according to Shariat Law because God is far higher than the one who may punish anybody twice for one and the same offence. Another kind of sin which can be hoped to be pardoned is the sin the compensation for which might not have been made in the worldly life but the offender had made a true Tawbah (repentance).

Mirsaad Or The Valley Of People’s Rights
The Third sin which is not at all forgivable is the taking away of someone's right or oppressing somebody. These are the rights of people. God will never overlook them. If one has taken unfairly or oppressively even a little trifling thing of somebody unjustly, Almighty Allah will make the oppressor repay it to the oppressed. That is why the meaning of the words Mirsaad in the verse 'Inna Rabaaka Labil Mirsaad' (Surah Fajr 89:14) is mentioned as the 'Valley of people's rights', that is to say; your Lord is there to make everyone's right repaid to him.

Suspension For One Year Because Of A Straw
Muhaddis Qumi has, in Manaazil-al-Aakhirah, recorded the story of a famous pious man Sulaiman Daaraai, the well-known worshipper. He writes that after he died, some persons saw him in their dreams and asked about his condition. He said: I have remained suspended for one year in a valley. The reason is that, one-day, some people were carrying bundles of grass to the city. I just took one straw from a bundle for using it as a toothpick. After my death, I am in this chastisement for the last one year for the sin of not obtaining the consent of the owner of the grass before taking up the straw from his bundle. They (the angels) ask me: Did you know that the owner was agreeable?

A Thousand Years In The Valley Of Oppressions
Do not ask: Is a straw too of any worth? It is true that its value is insignificant. Yet, it is a property anyway, of someone. Some people take up some things from the property of the owner before the latter's eyes (the owner keeps mum because of a regard or respect). Such taking away of other's property is called 'Akhz-be-Hayaa' about which it is mentioned in narrations that it is just like usurpation and hence prohibited or Haraam. Do such deeds that, while leaving this world, there may not be even the slightest burden on your shoulders.
If you were careless about the till now, still try to obtain the agreement of the owners of the usurped things. This valley of rights too has stages depending on the people. It is written that for some, such suspension will be for a thousand years.

Encroachment Of Others Rights
God Almighty will deal with people in two ways. One dealing will be based on Adl (Justice of God) and another based on Grace (Fazl and Karam). Everyone who has to pay someone's rights will be compensated by snatching away some good deeds of the oppressor and giving them to the oppressed one. This dealing will be based on Adl (justice). For instance, somebody had put a false allegation on someone or had backbitten him or her.
Now only God knows how many of the good deeds of the backbiter should be taken from him and given to the oppressed one. Now, in case, the oppressor has nothing like a good deed in his account, the demand of the Divine Justice will be that a proportionate sin of the oppressed may be added to the account of the oppressor or backbiter.

No Oppressor Will Escape The Punishment Due To Him
There is a lengthy hadith in Rawza-e-Kaafi. The hadith no.79 is regarding the accounting and meting of justice on the Day of Resurrection. Therein Imam Zainul Aabedeen (a.s.) is reported to have said: God Almighty says: I am the Only One God. No one except Me is worth worship. I am the Ruler and the Judge Who says nothing against truth. I deal with you on the basis of justice.
Today no one will wronged in My Most Supreme Court. Today I will take from the mighty wrongdoer the compensation of the oppressed weak. I will recollect from the debter the due debt and will make the payment of one another through exchange of good and bad deeds done by them. Today is the Day on which, from this difficult path, no one can pass from my sight without accounting for the rights which were usurped by him.
O people who have gathered in the Grand Field of Mahshar! Catch hold of every concerned oppressor and obtain whatever is due to you. I Myself am the witness against the oppressors.

Repayment Of Dues Of Believers From Deniers And Vice Versa
It is mentioned at the end of the aforesaid hadith that a Quraishi man asked the Imam (a.s.): If some dues are to be paid to a faithful by a denier what will he obtain from the latter who will be in hell?
The Imam (a.s.) replied: A proportionate cut will be made in the sins of the faithful (momin) and its punishment (azaab) will be added to the chastisement of the denier (Kafir).
The Qureshi inquired: If a Muslim has to obtain his right from another Muslim how will it be retrieved? The Imam: A proportionate cut will be made in the good deed (reward) of the oppressor and it will be added to the reward of the latter whose dues had remained unpaid.
Qureshi: What if there is no good in the scroll of the oppressor? Imam (a.s.): A proportionate cut will be made in the sins of the oppressed and its punishment will be added to that of the oppressor.

Cut In Punishment Equal To Right
It must not go unmentioned that if a Kafir has to get from a Muslim, since then, a denier is not qualified to obtain a believer's good, the demand of divine justice will be that a proportionate cut be made in the punishment due to the denier. For a further understanding of this problem please refer to the story of a Jew who had to be paid five Qiraan from a worshipper believer. If has been mentioned in this book earlier.

Fear Of The Claim Of Rights
Imam Zainul Aabedeen (a.s.) has been quoted in Layaaali-al-Akhbaar at p. 548 that on the Day of Resurrection, the hand of a person will be held and raised high so that all could see it. Then it will be announced: If anyone has to take his dues from this person he should come forward and take it. No other thing will be harder than this situation for the people in the field of Mahshar that one's acquaintances should see one. He will fear lest they may not claim their unpaid dues from him.

Most Poor In The Day Of Judgement
In the same book, the Holy Prophet (s.a.w.s.) is reported to have said to his companions: Do you know who is poor? They replied: He among us is poor who does not have any money nor any property or assets. The Holy Prophet replied: In fact, the poor in my Ummah is the one who will bring with him his prayers and fasts and Zakat and Hajj but he had also given a bad name to somebody, had taken someone's property unjustly or had shed somebody's blood wrongly or had beaten someone unlawfully.
So all of his good (rewards) will be distributed to the aforesaid claimers in proportion to their usurped rights until all of his good will be finished and yet he will remain indebted. Then the sins (punishments) of the rightful claimers will be thrown in the account of the indebted fellow.

Dealing Based On Fazl (Grace)
The person who will be showered with Divine Mercy, will, even though he will remain suspended in the station of people's rights, as he will have to repay some unpaid rights he will be dealt with gracefully. At the time some persons will be drowning in their own perspiration. The Most Merciful Allah will, then show to the gathering a palace of paradise. Then a voice will be heard: “O people! Who claim any right from this one of my servants! Listen. If you desire to dwell in this heavenly palace then, give up your dues and free him.
Truely, a man who behaves straightly with God, his dealing remains straight everywhere. How it is ever possible that God may not straighten it? That is why Imam Sajjad (a.s.) weaps: Waman…meaning: O God! Who except You will save and protect me tomorrow from the claimers of their rights.
Come, lest us all also pray: O Lord! Please deal with us on the basis of Your Grace and Generosity and Kindness: Illaahee…Yaa Kareem. The Hazrat had taught a prayer to Abu Hamzah Shimali in which it is mentioned at many places: Wa Ilaa… Meaning: My God! I beseech Your Grace and Mercy.