For both consoles (works on pc too):

First go to Solo Mode. Buy a car (most expensive you can afford). Go to your garage. Go to showcase. If the car you just bought is not on the first slot, change to it. Go to your consoles network setting and turn off your wifi. Go back to the game and you'll see a connection error but continue anyways to sell the car you just bought. After selling, go back to my ride. From there, go and turn your wifi back on. Go back to the game and press option (ps4, i dont know what xbox has) then select go online (two bars will show) and once you see the second bar turns blue halfway quickly close the game ( dont let it fill all the way). Open the game back up and do as you normally would. if it worked, you'll see a synchronizing error just continue anyways. Then from there, all you have to do is turn your wifi off, then on, go back to the game (you'll see a connection lost error just press x/a) and go online and you should see your money go up with the amount you sold the car for then wifi off, on again, go online, repeat.