How we change what others think, feel, believe and do
Where the other person says something has happened, disagree. Deny that it happened.
It is easier to deny something about yourself. In particular, you can always talk about how you feel with complete authority. Nobody can tell you with full certainty what your emotions are.
Take issues with small points and assert that they are not true. Or chunk upwards and show how the bigger picture is not right (or how the detail does not match the big picture).
A weaker form of denial is to express uncertainty about what the other person says.
Sorry, I did not agreed to any of that.
Mmm. I'm not sure if that is entirely correct. Could I add some useful extra information?
I was not here last week. Why do you say that?
Denial is effective calling the other person a liar, or at best saying that they have a faulty memory. It will often put them off their path of persuasion and give you time. When you are in the position of the buyer, you can often do this without worry of serious argument, as the seller will want to keep you sweet.
Denial is a strongly assertive act, and wrests control off the seller, putting you in charge. In the pause that denial creates, you can now head off in the direction of your own choice.
This is a particularly valid approach when the other person is using an assumptive method, presuming that what they are saying is true even though there is insufficient data to prove this.