I found when I was looking at buying my Alfa that there were two cars, same spec, one had 55k on the clock and the other had 85k on the clock, the 85k car was a much much cleaner example, sat next to each other at the dealership. the lower milage car had badly worn seat, dings all over the dash and scuffs on the wheel / knob and stunk of fag smoke - so the previous owners make all the difference!
In all honesty, there are a lot of very good Alfa 159s out there and you want to buy the right one, replacing the gear knob isn't easy as it's a lift up reverse, so you'd be as well buying the entire selector assembly from a breakers and doing it that way.
If that's what they've agreed to then you need to hold them to it or walk away, they've breached the contract in not completing it, so you're perfectly entitled to your money back plus costs if you really wanted to try it on, but chances of that are slim.
Walk away, I wouldn't be surprised if they suddenly have a change of heart....