Does charity purify ill-gotten gains when the amount is unknown, or is a specific intention required to purify ill-gotten gains?

Purifying the unlawful portion is done by extracting what is most likely to be the profits generated from the shares, equivalent to the proportion of unlawfulness in the company's activity. This is not charity, but rather a disposal of unlawful money to clear one's conscience. If you know the proportion of unlawfulness, you extract it; if you do not know, you exert effort based on what is most likely to be the case, keeping in mind that the unlawful proportion applies to the profits, not to the principal capital.