How to reply skillfully when someone asks you to treat?

If you are a good friend and are willing to invite you to dinner, just say generously: OK, let's go to dinner together.

If you don't want to be invited, just say humorously: OK, it's my treat, and you are the host. Or you can say humorously: My stomach tells me that it is not hungry, but it is thirsty. Why don't we go and have some water together? If this person is not your good friend, you can say in a perfunctory way: I have something to do now, and I will invite you to dinner next time.