You definitely found a bug! Can you explain how you opened the P.I. bit without doing the Gretta photoshoot? Did you earn money all the way up to 5K by doing other stuff? If so, this is a design bug on me, 100 percent. If that isn't how you got there, please let me know, and it's likely a coding error.
Anyway, to get around this, you have to go back a bit, unfortunately. If you can load a save around the time Dex asks for help, then follow the storyline on getting Gretta's photoshoot... if you save Dex, go talk to him again the next day or so after talking to the gangsters... you will earn money for Dex, to earn the building for yourself... if you kill Dex, it's a job you do and you get to keep all the money (5k).
Thanks for finding the bug and sorry you had to deal with it! I am looking into several ways to fix this. May take a couple days.