Du Plessis was charged with an article 2.2.9 offence of the ICC Code of Conduct which relates to "changing the condition of the ball in breach of Law 42.3 of the Laws of Cricket, as modified by ICC Standard Test Match, ODI and Twenty20 International Match Playing Conditions clause 42.1".
The charge was laid by on-field umpires Ian Gould and Rod Tucker, third umpire Paul Reiffel and fourth umpire Shozab Raza after the end of third day's play.
The TV Umpire brought this to the notice of the on-field umpires who, in accordance with Clause 42.1.1 of the ICC Test Match Playing Conditions, which deals with The Match Ball - changing its condition, replaced the ball, awarded five penalty runs to Pakistan and reported du Plessis.
Before the start of fourth day's play today, David Boon of the Emirates Elite Panel of ICC Match Referees handed the fine to du Plessis who had pleaded guilty yesterday evening.
"After discussions with Mr du Plessis, he has elected not to contest that charge, but I am also satisfied that this was not part of a deliberate and/or prolonged attempt to unfairly manipulate the condition of the ball, and that the imposition of a fine of 50 per cent of his match fee is appropriate considering the circumstances."