Addendum to contract for deed?

I have a question regarding the legality of a situation I have going on as the buyer in a C4D contract.

On 12/9/10, we signed an addendum with the seller stating that we would pay $1010/month, be charged at 7% interest, a 3 year initial contract and that we would pay him $8000 from our first time homebuyers credit as part of the down payment. This is not a notarized document

On 3/21/11, we closed on the deal at a title and abstract company. The new paperwork then stated this:

7.5% interest, $1050/month, 5 year deal and $8000 deferred downpayment due by 4/1/2015 (The end of the 5 year deal) All these documents, including the promisory note, are notorized and witnessed.


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