Co-parenting issue - just need to vent

My ex and I have a daughter together (9), but have been separated since she was 2. We have both moved on and are married now. We don’t have a legal custody agreement, but I have her all the time, except every other weekend. I also let him take her if he wants extra time and try to keep him involved. He (is supposed to) pay me $150 a month, and is currently about $600 behind. I think it’s pretty pathetic considering $150 is NOTHING.

Anyways, every summer my parents have “grandkid week” where her and her cousins all get to spend he week with grandma and papa and do lots of fun stuff (amusement parks, swimming, art stuff, etc.). Earlier this week my ex’s wife texted me and asked if they would be able to get my daughter on Thursday morning instead of Friday evening, which is what we normally do. I told her about what week it is with her family, and that it wouldn’t work out. She responded “ok” and we left it at that. Fast forward to tonight, my mom called me and said that he texted her to see if he could get her on Thursday at noon - AFTER I had already told them it wasn’t going to work out this week. It makes me so unhappy that he is trying to go behind my back. We are definitely not in one of those situations where I try to “keep her from him”, it just doesn’t work this week. Of course, when I try to call him and talk to him about it, he doesn’t answer. He never does. I’m so annoyed.