As others have said, drop the rope. Call their bluff with a "That's too bad. We'll miss you at dinner! Maybe we can try to get together some other time." After reading the other thread, part of me wonders if they like the attention of making the plans with you and then canceling and having you chase after them. Four times? Really? Who DOES that?
And my first thought was that this was *your* present. YOU should be the ones to use it, without any obligation to take anyone else. There's no need to spend it on other people! It takes away from you fully enjoying your own gift, especially when you have a contentious relationship with the people you're inviting.