Question

Hello Rabbi,
 
The supplier and the buyer discussed the quantity of goods and the price. The supplier offered quantity X for 3Y shekels, and quantity 2X for 5Y shekels. The buyer said he was willing to take quantity X for 3Y, but quantity 2X only for 4Y. There are written documents with these details, and these were not just ideas raised during the negotiation stage, but actual offers.
 
In the end, they closed a deal for quantity X at 3Y.
The supplier mistakenly delivered quantity 2X.
After the mistake was discovered, the supplier said to the buyer: come give me 4Y, as you originally wanted.
Returning the goods to the supplier is not worthwhile for him.
But the buyer says: I won’t give you 4Y — maybe I won’t give you any money at all for the extra X, and maybe you’ll get only a token amount from me.
Because you delivered it without an agreement, I’m not obligated to give you anything, and now you’re stuck, so lower the price for me even more, or if you want, come take it. (The buyer knows that it isn’t worth it for the supplier to come and take it back. But maybe just out of anger he will come and take everything.)
What should be done in such a situation from the most basic standpoint of Jewish law? What is one obligated in and what is one not obligated in? Telling people, listen, this isn’t nice, be righteous and do ABC, and in return God will bless you with abundant livelihood and health — that works less well.
 
Thank you very much.