He wants to get out of the investment fund but they stipulated that he must leave some money in it

Praise be to Allah.

Firstly: 

It is not permissible to subscribe to the investment fund that gives a fixed percentage of the money deposited in it, because this is riba which Allaah and His Messenger have forbidden. This is no different from the riba-based banks except in name only, and you have done well to get out of it. 

It is well known that one who repents from riba is entitled to his capital only, because Allaah says (interpretation of the meaning): “if you repent, you shall have your capital sums. Deal not unjustly (by asking more than your capital sums), and you shall not be dealt with unjustly (by receiving less than your capital sums)” [al-Baqarah 2:279]. 

The one who has taken riba-based interest has to get rid of it by giving it to charitable causes. You have done that with half of the interest when you gave it to the mosque. As for the other half which you gave to the fund, it seems that this is also permissible, because this wealth in fact belongs to the fund, since you took it by means of riba, but riba is not a valid reason for you to take possession of the interest; you are entitled to take nothing but the capital of your wealth only. 

Secondly: 

If the matter is as you describe, that the fund stipulated that some of the money must be left in the account, then there is nothing wrong with you leaving some money in the account, because you were compelled to do that and did not do it by choice. But you have to understand that you have no right to this money, because the money that you have left in it is riba and does not belong to you. 

So it is not permissible for you to ask the fund for any money after today, rather you should consider yourself to have finished dealing with it forever. 

We ask Allaah to compensate you with something better. 

And Allaah knows best.