If Im understanding your idea correctly you want to provide potential customers who for some reason did not qualify for a home an incentive to direct people to you.
IF this is the case you have a few options but each you need to be careful with So ill give you the pros and cons of each
Percent of commission. .... you could be asking for more work as far as tracking sales and taxes if it is applicable.
If you use a contest most customers dont care to go that route because they feel there is no chance of winning.
My best suggestion is to have a set amount per sale and do giftcards.
One, you can track your giftcards. You then know how much is going out and a better tracking method can be established as to how well this referral method worked compared to leads and sales.
Hope this helps and I wish you luck in you business.