The thing is, you asked for suggestions. You didn't say 'go ahead, buy it now and I'll reimburse you'.
So, for going ahead on the assumption that you would buy / pay for what he asked for, your DF was a bit of a cf actually. Especially if he used funds in a way that caused problems or worry for your DM.
Presumptuous behaviour on your DF's part.
Had he offered the idea, as asked, you could have bought that, or something else, as your budget allowed, closer to Christmas.
I do think you'd be reasonable, in the circumstances, to say 'actually, I only asked for ideas, my budget changed. I now can't afford £20. I misunderstood and thought you two were doing a nice thing by buying the games 'from me', for me.'