My ex has asked me to go halves on weekend day and overnight childcare on a weekend in August. The weekend has been in the diary as his "access" time with my DD since about April as I am going away with my new partner over this weekend (otherwise I am usually quite flexible). At the time I told ex I wanted this weekend off he said this was fine. However since then he has been invited to a wedding of a work colleague and he really wants to go. He has now asked me to pay halves (the cost will be just under £100 each which is far too much for me anyway) for the childcare seeing as we are "both unavailable".
I have told him no, as this has been in the diary as being "his" weekend with DD for ages. In the past, when I have wanted to do other stuff on "my" weekends and I know ex is unavailable I have arranged my own childcare actually not expecting him to help out as I consider it my responsibility.
AIBU?