This is the sort of thing that would piss me right off. If he wanted a physical present to give to her then he should pay the full amount, and say so upfront instaed of agreeing one thing and doing another. I doubt he meant any harm, but he probably can't see that it could be dissapointing and unsettling for his dd.
The problem is not that he bought the thing, it's the fact that you will have been there talking to dd about saving, told her that you had actually spoken to her Daddy and made an agreement, and then he comes along and pisses all over it.
So the message dd gets is that the two people that are most important to her, talk about her, about something very important to her, they appear at first to agree and be saying the same thing, but then it doesn't work out like that. And Mummy has said something about Daddy that's not true.
The message she needs to be getting from both of her parents is that they can talk about her and agree on important things, that they will support eachother, and both of them agree what is best for her. Otherwise it is just confusing and unsettling for her. She needs to be able to believe her parents when they say things about the other.
Ok, I'm ranting a little now, can you tell I have had simelar things to deal with in the past? 
I've had to emphasise to me ex that we both have to appear to be working together for the sake of the dc, and now that we do that it's much better. There was a phase where ds1 especially, would try and play us of against eachother. But when we started working together more, and supporting the other even if we don't completely agree with what the other has done, the dc have felt much more secure.