Question Summary:
Investing haraam wealth in stock exchange and later purifying it by giving the haraam amount as charity

Question Detail:

Asalamo alekum.  I had been working at a bank sometime back, but now I have quit and try to be mindful of halal ways of earning. However, I do have savings from the bank income. I have read that haram income can be purified if it is invested in a halal venture and the profit equivalent to the initial investment is given away as sadqa. Also, that the future profit from this investment will be halal.
 
For this purpose, can I invest in stock exchange and aim to earn profit through capital gains?  Will that be right path in purifying my income according to shariah?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Brother-in-Islam, what work were you doing at the bank?
Kindly clarify why you assume your earnings to be haraam.
And Allah Ta’āla Knows Best
Aboobakr Siddeeq bin Mufti Amjad Mohammad
Student Darul Iftaa 
Azaadville, JHB, South Africa
Checked and Approved by,
Mufti Ebrahim Desai.