Although I would definitely discuss with your Realtor regarding your specific circumstances, it is not uncommon to have addenda drawn up to clarify an issue, to make a point, or to resolve an open question.
In most cases Realtors have forms approved by their state body (and their attorneys) to deal with common issues, such as a seller making repairs as agreed based on an inspection, or a change in the escrow instructions such as a delay in the closing date.
Always get it in writing, and always send it through escrow.
Addendums are a normal aspect of a "purchase contract" that only become part of the contract when agreed to by the parties involved. Once you have an agreement only mutual agreement can alter the terms.
Since we do not have the benefit of the agreement, it may be best to seek the advice of a real estate attorney on this matter.
Gary is absolutely correct ... we would need to see original contracts to provide any advice or guidance. Most contracts do have safeguards for buyers in the way of inspection contingencies, etc. The answer to your question likely likes in the answer to this question ... Was the addendum in relation to an inspection or other contingency in the initial contract? Was it an Addendum, or an Amendment? Many use that word interchangably, but they are very different. One adds ... one changes.
Hope that helps, and please provide us more info if you wish.
Derek Bauer, Associate Broker / Realtor
Real Estate One