Total surplus in every transaction in constant; your value of what you are acquiring is above that of what you are giving up, as in your partner's, and that's why you engage in the transaction. These values don't change with price; the price simply determines how the surplus is divvied up. Nonetheless, you still want a greater share of the surplus for yourself.
Today at Starbucks, I bartered my grande hot chocolate (no whip cream) down from $2.84 to $2.70. I therefore have deemed August 2 "Surplus Day" as I'm rolling in the additional surplus.