Sharp's brother william held a regular surgery for the local poor at his surgery at Mincing Lane, and one day in 1765 when Sharp was visiting, he met Jonathan Strong. Strong was a young black slave from Barbados who had been so badly beaten by his master, David Lisle, a Lawyer, that he had been cast out into the street as useless. Sharp and his brother tended to his injuries and had him admitted to Barts Hospital, where his injuries were so bad they necessitated a four-month stay. The Sharps paid for his treatment and, when he was fit enough, found him employment with a Quaker apothecary friend of theirs. In 1767, Lisle saw Strong in the street and had him kidnapped and sold to a planter called James Kerr for £30. Strong was able to get word to Sharp, and in a court attended by the Lord Mayor and the Coroner of London, Lisle and Kerr were denied possession of Strong. They instituted a court action against Sharp claiming £200 damages for taking their property, and Lisle challenged Sharp to a duel—Sharp told Lisle that he could expect satisfaction from the law.