From watching the show as a kid, I was always under the impression that the initial cash offer was scripted, but I think that, if a contestant became so sure he/she didn't want to buy it, then Jim would take over and try to convince him/her to go for it.
I can remember one episode where there was a champion who had won for I think 4 games and he NEVER bought an Instant Bargain. So, Jim went through the motions and offered him $500 to start off with, and, when the guy gave his usual "no" shake, Jim walked over to the contestant's podium and laid down $2,000 for him to consider. The way it looked on camera, I believe the $2,000 offer was all Jim's doing.
Even though Jim could've done parts of it on his own, I'm sure the producers gave him a limit as to how high he could go with the cash reward or how low he could drop the price.
That's my two cents, anyway!
Anthony