If the contract say they guarantee 8% per year then the scheme is that they rent it out for way more than 8% per year but that's what you get. Usually works. I've had agencies offer me 5% up front while they are renting out for 15%.
To add: With current offshore interest rates , if your money is offshore, where you can get a non-risk 4-5% , 8% is a bad deal. What I said above was several years ago when interest rates were 0%.