There’s a glitch in GTA 5 that will allow you to buy a property in the game for free! However, there’s a condition for it, you must have enough money to buy the property first. And also there are 2 kinds of property: business property (gives income) and non business property.
For the business property, you will get some presentation clip / movie when you buy the property. To buy it for free, right after you bought it and when the clip is playing, press start and replay some mission (preferably the short one), and when you’re done with the mission, you will be standing in front of the property, owning it, and get your money back.
For the non business property, you usually won’t get the presentation clip / movie when you buy it. So to buy the property for free, you need to use your car in here. Drive your car near the property sign / for sale sign, out of your car, buy the property, and after buying, immediately go back in your car before the game take your money, and voila.. the property is yours now for free.