Could I ask what the present is going to be and whether you bought it with your own money or whether you pool all your money with your wife?
If it's a sports car or an aeroplane then obviously you need to discuss it with her. If it's a jigsaw puzzle or a small toy then it really isn't significant enough to discuss anyway.
Also, doesn't your wife have friends she gives presents to without asking you first?
The trouble is if you make a point of telling her it makes it a bigger deal than it really is. It's hard for me to see why she needs to know. Complete openness can sometimes be insensitive. The other issue is, if you don't give your yf a present for his birthday how is he going to feel?