Had this problem and searched online for solutions but failed to find any, despite many asking about it. So I tried several times and found a solution. It has something to do with the online stock market feature. The following steps worked for me.
0. Load file before the assassination.
1. Before the job, buy Fruit stocks as usual.
2. Now log out from PSN (you can do it in game) and start the job
3. Finish the job
4. Now, watch your portfolio closely while waiting.
The key is: DO NOT log back in to PSN! once you log in, the stock price jumps right back to 0% movement and goes up again and stops at 25%!
So, check your portfolio via 'My Portfolio' link on the LCN page instead the BAWSAQ one - the BAWSAQ page won't work, as you are not on PSN
5. Now, the stock price will go up to around 50%. You can click 'sell all' in 'my portfolio' on the LCN page.
6. Log back in to PSN if you want to (the next stock Vapid also in BAWSAQ market)
Maybe there are other ways to do it, but this one works for me. Hope it helps.