AGREEMENT: Do You Agree to Disagree?

Good evening ladies and gentlemen,
I think you remember I made a post about agreement few days back. Do you know what I have come to understand about agreeing? Agreement must be decided on without any form of breaking of the rule of agreement after the decision has been made.

No breaking of the rule of agreement after the decision has been made!

My Recent Experience with lack of Compliance with the Rule of Agreement
Do you know what happens when one or any one of the parties involved in the agreement fails to comply with the rule of the agreement? Well, things end up really bad.

I recently got an experience of that fact. It was a real eye opener of the truth of that fact. I would tell you more about that in a jiffy.

(Please, don't just read; read and understand the lesson in the story)

Here is the story in details....

Two people agreed on a business deal. But, the business wasn't really sealed as the persons involved did not realize that the business deal was without real agreement.

The lady decided to sell her an android phone to the guy, who agreed and then left with the phone without an open agreement of how much he would buy the phone. After he left with the phone, she realized that the agreement wasn't really sealed. She went back to him to ask for the money, he gave her 500, but she was upset because she was expecting more since she had hoped to get more since she thought he would be paying 15,000. However, he had agreed he would be paying 4,000 for the phone. So, in the end, both of them didn't have any agreement. Well, let me say they add "no agreed agreement"

Let me stop here with this question:
Which of them was wrong?