I believe Roux stated after reading the Steenkamps missive about the payments that there had been no conditions whatsoever for these from the defence's side nor for the lump sum that was offered so I really can't believe as an officer of the court Roux would have said this in front of the Judge and Nel if it were a lie and less still in front of the Steenkamp family and their lawyer since he could be reported and easily find himself disbarred for something as serious as that.
Of course such a deal would be very embarrassing and as you say cause a serious problem. My point is that there could have been more subtle ways in play. Pistorious side could have simply expressed concern about giving money to someone who may speak out against them. The Steenkamp's side could have hinted that such an agreement might 'oil the wheels'. In other words implied not openly declared.
After all it was included by somebody or it wouldn't have been in the statements about the money. Think about it. Either way the Steenkamps were badly let down.