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 incident that led to the charge being laid took place in the 31st over of Pakistan's second innings on Friday afternoon when du Plessis was seen on television rubbing the ball in the vicinity of the zipper of his trouser pocket.
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 on Saturday, David Boon of the Emirates Elite Panel of ICC Match Referees handed the fine to du Plessis who had pleaded guilty on Friday evening.
Commenting on his decision, Boon said: "I am satisfied that the player's actions warranted the umpires applying clause 42.1.1 of the ICC Test Match Playing Conditions, including the laying of a charge under the ICC Code of Conduct against du Plessis in respect of changing the condition of the ball.
"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."
Penalties for offences included in Level 2 of ICC Code of Conduct can range from 50 per cent to 100 per cent of a player's match fee and/or two suspension points, which translates as a suspension of one Test, two ODIs or two T20Is.
--IBNS (Posted on 26-10-2013)