Bum.. Just typed a long reply and lost it grrr....
Anyway.. I have a DC with my ~XP. I get paid a small amount monthly (still more than the OP though). My DH has two DC, he pays his XP a lot.
It isn't fair IMO. However, I have learnt over the years to just suck it up. If the other party is being a knob about it there isn't any thing to be gained by getting upset about it. It truly is not worth it.
My XP pays his small amount - which is no way covers 50 % of DS's costs. Even school trips, music lessons etc are to be funded from that. I do ask for more towards this extra things but it falls on deaf ears. He never, ever takes him on holiday or for an extra night - nothing. This makes me v grrr... I send him a list of school holidays, and what we have planned but he takes all his work holidays in term time in order to take his young, pre-school DC on holiday - meaning that my DC can't go away as not allowed to take time out of school.
DH pays his XP quite a lot IMO. In addition to the CM which is above CSA levels he contributes 50% of school uniform, school trips (incl £900 ski trip!), private optical treatments - don't ask - and whatever else crops up. Also we take them on good holidays and a lot of days out. Oh, and buy them clothes as and when requested. We also have them on extra nights - more than the calculations for CSA dictate.
So - in effect my DH is supporting quite a lot of families.... (esp as his XP hasn't worked for many years - but complains about having no money.. Humph!_
I have come to the conclusion though that there is just no point in letting it bother you. It is a temporary problem - the DC will all grow up at some point and will be self supporting.
The money is important - yes - as is the attitude of the parent who pays the ~CM - but if they are being a cock there is nothing to be done. Yes, you can go through the CSA - and may be get more money. But it is in situations like this when knobheads show their true colours.
OP - don't give him the £20 for the holidays. If he short pays you by £20 threaten him with CSA re calculation. In fact - I would do that anyway.
I hope you work something out - as it just isn't right.
x