The perfect free solution I have found

IT works great with 3.9.1
Never expected to be so easy.
The user sees below the price a box where he enters his price (you can put whatever text you want in front of that box) and then he clicks ad to shopping cart and then all is finished.

Also some tips:
1. Setup WooCommerce so no user can buy as guest, he needs to create account. This helps him see his past offers in the case they were refused, and it also makes you have complete details on the buyer, for later shipping.
2. There is an option inside WooCommerce when you set up payment by PayPal to chose not send charge but to only send with attempted payment. This way, you can refuse the order and refuse his payment. This way you make sure they will pay when they make the offer.

Best solution I found for free. Thanks