If you ask this for the clones challenge, we are not allowed to use Edit Town to move Sims out. We have to do it with the "move" interaction on the cell phone or computer.
Then, you can only buy a house worth less than the cash funds of your active household. Let's say they have 50.000$ in cash. You won't be able to move a Sim into a house worth more than that, and if you pick up house for 20.000$, they'll be left with 30.000$ when the Sim has left.
Give it a try, you'll see it's much more simple in game. Just remember that you have to do it in Live mode, and you have to select a house, not the kick out option.
