You are the Chief Arbiter at the event.  The following happens during round 4 on board 2 of the Open section.  It is White to move.  Black is not at the board (he went to the bathroom).  White makes his move with a bishop but before he completes the move, he realises that this will result in him being checkmated.  He places the bishop back and moves another piece and completes his move.  None of the arbiters saw what happened.  Only the players playing on board 3 saw what happened.  Eventually White wins the game.  After the players on board 3 completed their game, they come to you and inform you what they have witnessed.

         You approach White and asked him if this is true. 

         What will your ruling be in each of the following cases: (3)

         a. White denies that he touched and moved his bishop.

        b. White admits that this is what happened