How to dispose of haram wealth after repenting
Question

I am an Accountant employed at an accounting firm. I prepare financial statements, tax returns and advise clients on financial & tax related issues. Our clients are mostly small restaurant owners. We also have some clients in the real estate & auto dealership business. The restaurant owners, among other things, sell pork products. All the clients deal with interest (pay and receive). Sometimes I have to write letters stating the clients financial status knowing that this letter will be used to take out interest based loans. Is my work Halal? If it is not and I leave this job for something else that is Halal, can I keep and spend the money I have earned and saved from this job? Can I invest the money in a business? Can I go to Hajj with this money?.
Praise be to Allah.Firstly: 
Working in the field of registering and calculating riba, or writing letters concerning it and other things that amount of helping with it, is not permissible, because it is cooperating in sin and transgression, and Allah says (interpretation of the meaning): 
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allah. Verily, Allah is Severe in punishment” [Al-Maa’idah 5:2]
So you have to give up working in this field and limit yourself to permissible kinds of work. Whoever gives up something for the sake of Allah, Allah will compensate him with something better than that.
 See the answer to question no. 59864 which explains the prohibition on helping with riba, even if it is only writing letters mentioning it. 
Secondly: 
When a person repents to Allah from doing haram work, from which he has acquired wealth, such as payment for singing, bribes, payment for fortunetelling or bearing false witness, payment for recording riba and other kinds of haram work, if he has disposed of that wealth, he does not have to do anything. But if the wealth is still in his possession, then he must get rid of it by spending it on charitable causes, unless he is in need, in which case he may take only as much as he needs, and he should get rid of the rest. He is not allowed to use it for Hajj, because Allah is Good and only accepts that which is good. 
Ibn al-Qayyim (may Allah have mercy on him) said: If someone else gives him a haram payment and he accepts it, such as a prostitute, singer, wine seller, one who bears false witness and the like, then he repents and that payment is in his possession, then according to one group (of scholars) he must return it to its owner, because it belongs to him and he did not take it with the permission of the Lawgiver and the one who gave it did not get any permissible benefit in exchange for it. Another group said that his repentance means that he must give it in charity, and not give it to the one from whom he took it. This is the view favoured by Shaykh al-Islam Ibn Taymiyah and it is the more correct of the two views. End quote from Madaarij al-Saalikeen (1/389) 
Ibn al-Qayyim discussed this matter at length in Zaad al-Ma’aad (5/778), where he stated that the way to get rid of this wealth and complete one's repentance can only be by “giving it in charity, but if he is in need of it, he may take as much as he needs, and give the rest in charity.” End quote. 
Shaykh al-Islam Ibn Taymiyah said: If this prostitute and this wine seller repent, and they are poor, it is permissible for some of this money to be given to them, as much as they need. If he is able to engage trade or a profession such as weaving or spinning, he should be given enough to form capital for him."(Majmoo’ al-Fatawa  29/308) 
Thirdly: 
From the words of Shaykh al-Islam Ibn Taymiyah quoted above, it may be understood that if the one who repents from haram earnings is in need, he may take from that wealth as much as he needs, and he may invest some of it in capital for trading or skilled work, then whatever is surplus to his needs must be given in charity. 
Fourthly: 
As some of your work is permissible and some of it is haram, try to work out what proportion of it is haram, and get rid of the corresponding amount of the wealth that is in your possession. If it is too difficult for you to work it out, then get rid of half. Shaykh al-Islam Ibn Taymiyah said: … If halal and haram wealth are mixed, and the proportions of each are not known, then divide it into two halves."(Majmoo’ al-Fatawa  29/307) 
And Allah knows best.