From what I gather from your question... you made an offer (I'm guessing not full price) on their property which they then signed all the changes except for price and sent it back?
If this is the case your agent should have realized the mistake and contacted the selling agent to have her clients initial any changes that were made. This saves and potential issues later within all parties. Such as from seller to their agent (you didn't properly discuss this offer with us), or between yourself and the sellers.
If this doesn't answer your questions please post another thread to this question(answer this question) so I can better assist you.