What is free Duke's reply to brabantio's complaint?

At first the Duke is supportive of Brabantio's complaint.

"Whoe'er he be that in this foul proceeding

Hath thus beguiled your daughter of herself

And you of her, the bloody book of law ....."

Indeed the Duke would have liked to have a say in such a high profile union like Othello and Desdemona. Othello's explanation, however, wins the Duke over. Othello says that he won Desdemona not by witchcraft but loved him "for the dangers I had pass'd, And I loved her that she did pity them." This is the same reason that the Duke and nobleman love Othello so much as well; he wins wars for them.

Othello's contribution to Venice much outweighs this indiscretion. In the end, the Duke agrees that what is done is done and let it pass. , "To mourn a mischief that is past and gone Is the next way to draw new mischief on. "

Ps. I'm not sure what you meant by "free" but your question looks similar to one I answered a long time ago.