Gift her the clap, since she thinks it, make it true....

Maybe she just likes to be reassured. Girls love attention even though it's hard for you because of your job. Sorry dude :

If you are spending long hours away from home, such that you barely have time for friends, much less an affair, as you put it, you probably have little time to spend with your wife. The fact that you spend so little time with your wife--and, for the record, "working late" is a common excuse employed by cheating husbands--has got her suspicious as it is. Then the money goes missing--I assume she handles the day-to-day running of the household--and she gets even more suspicious. If you haven't been the sort to give her lavish presents on her birthday, she might not have made the connection. Heck, she may have thought that you were going to give her a totally unpleasant "surprise" on her birthday. So, you have to reassure her. "I believe someone has a birthday coming up." Spend a little more time with her and less at work. Blah, blah, blah.

She's gonna feel like a douchebag when she finds out it was for her.

Could be either she is actually cheating on you, (common if they are accusing you) or she is feeling unloved as if you work that often you obviously don't have enough time for her which may have caused her to cheat (not that I condone that in any way) either way, I think you two need some time together to talk and rekindle your marriage. If she's in fact cheating however, I'd leave her and go to Bermuda by myself :)

This is where you need to just show them that you aren't cheating on them so it doesn't escalade to something bigger and then be mad at them for ruining the surprise

Seperate accounts!

For you idiots. by work 24/7 usually means on call 24/7. like tow truck drivers etc

Motto be rude or anything but Maybe your wife is having an affair and that's why she's blaming you ?

Are you not allowed to spend any of the money that you earn?