Dedicated to Paty, a friend of mine.

No english version because I can translate it but I don't know if the expression is even used that way. Explanation:

There's a saying in spanish that reads "Que Dios te lo pague", roughly translated to "May God repay you(r kindness)". I believe in english is a saying that means a good wish to others. A one's wish for people who are good or kind, to receive the same ammount of goodness and kindness from God's mercy. Now, for spanish speakers, I think that was the same at first but, as generations passed, we started using it in a more literal and ironically sense. When someone makes a favor for you or, even better, lend you money, you are in debt with that someone, altruism of that person aside. So, we say "Que Dios te lo pague" implying in a funny tone that we *do not* intend to pay our debt or repay said generosity by lending the responsability to God. Yeah, kinda mean if I put it that way, but it's worth mentioning this is the type of thing you say to people close to you (like good friends, family, etc) and you don't really mean it.
Now, Paty did a small favor to me and Byakuren is his fav 2hu char. Since this char is also treated in the fandom as "Youkai Jesus" sometimes, everything tied up together and I said to me "I have to make this joke". I changed "God" in the saying for "Myouren", because I thought it would fit Byakuren better and that's what you read in the pic. "Que Myouren te lo pague" (May Myouren repays you) would be Byakuren's way to say "Que Dios te lo pague" since she doesn't want to repay you anything. But she will, in the end.

And that's it! Hope it makes some sense and sorry for any troublesヾ(￣▽￣)