In that case the employer is going to hear " I want more money because I think you owe it to me". It is very easy for the employer to respond with "We don't feel the same way because X,Y and Z reasons or we cannot afford to pay you for the work you did for free".

Negotiations are about leverage. Having a good relationship with someone is great. But they are not going to give you a big chunk of money because the like you. They are going to give you a big chunk of money because they need to you stay on when you are on your way out the door.