I am intending to give this money as sadaqah to one of my relatives as he is about to get married, is this permissible or not? He asked me to lend him some money but -unfortunately- he does not have enough to return it to me.
In the answer to your first question (no. 96614) we stated that whatever of this money is still in your possession should be spent on charitable causes, and you should not make use of it. In that question we referred to another question, which says that is it permissible for the penitent to make use of some of the haram money if he is in need. See question no. 78289.
Undoubtedly one of the ways of doing good is helping the one who wants to get married so that he can remain chaste. Based on this, there is nothing wrong with giving this money or some of it to your relative who is going to get married.
This is not regarded as charity or zakaah on your part, rather it is ridding yourself of haraam wealth. As for charity and zakaah, they can only be given from halaal wealth, because Allaah is Good and does not accept anything but that which is good.
We ask Allaah to help you and guide you.
And Allaah knows best.