Name changed as this is very identifying.
XH and I split 11 years ago, both remarried, both have other children but we have a DD (14) together.
We haven't spoken face to face in years not because we hate each other but because we just don't have to. So decent but kind of cold relationship, we're like 2 strangers raising a child together.
XH pays the minimum CM that he has to pay, which is £170 a month for DD and has been doing so for years, although actually getting the payment was such a hassle. He hasn't missed a single payment since he started paying about 8 years ago.
This Christmas DD is all set to go away on a ski holiday with the school, the cost was around £700 pounds, XH has another DD with his wife who has just decided that she wants to go away on a geography trip with her school over February Half Term, the deadline for full payment is Mid October.
XH emailed this afternoon saying what a wonderful experience this would be for his DD2 and that they currently can't afford to send her as he's paying me £170 a month, so would I forgo payments for 2 months so that like her older sister, his DD2 can go on a school trip as well. 
First of all, he's not paying me £170 a month, it's for his daughter and secondly what??
To me it reads as if he wants to stop financially supporting his eldest for 2 months, so his other child can go on a trip?
I know some of you will say it's only 2 months but that £170 is DDs travel money, after school activities, weekend spending, a small portion goes into her savings.
But as he's written it, if I say no, his DD2 misses out? Surely that's not my problem or Am I just being unreasonable