Relate the very odd incident that Enfield tells Mr. Utterson?

Enfield tells Utterson about a very strange occurrence in which he was walking through twon and witnessed a man trample a little girl in the street. The man was apprehended, and the girl's family, as well as the witnesses involved decided that in lieu of calling the police they would demand (blackmail) the man for a total of one-hundred pounds. The man came back with ten pounds in gold and a cheque....,, this is where it gets strange, as the check was signed by Dr. Jekyll.