Let's be honest. I bet the king knows this. He showed how much he investigated before an encounter in person. There is no way he didn't know how this pig man got a half price.
He could make a silent retreat. But, He let them know that there is 'alternative buyer' there. So, they can deal with it in future. It's kind of test to see how they handle and worth to do bussiness together. (I might read too much into it)
Unless, Author is aiming for the king to lose their face to gain more cooperation to demon lord group. But, that will be too easy and a bit out of king character, I think.