First, there is a lot of 'legal' being thrown around and if you are looking for legal advice, Trulia is the WRONG place.
Next, regardless of what a buyer and seller agree to if it violates Fannie Mae or underwriting requirements and you don't get the loan you have a whole new predictament on your hands.
It sounds like you have neither agent nor attorney since you are asking these questions on Trulia. There is absolutely NO WAY anyone here can give you good sound 'legal' advice for two reasons, first your question lacks way too many details next were not attorneys.
Lastly, please don't try to piece mill together a contract based on random advice you are getting online. Employ a licensed professional and/or attorney.