Be careful! WD-40 is a lubricant. It's very greasy and is used for loosening rusted bolts, squeaky hinges, things like that. It is suppose to break up the ink so you can wipe and lift off the ink but I never tried this. Because your leather on your wallet is coated and i don't believe your leather porous so that may work in your favor but BE CAREFUL! You know before you try, let me try it on a piece of scrap vachetta leather. Vachetta is porous as it is raw so the result won't be the same as your leather but let me test it out. Im sure the vachetta will drink up the grease and possibly the ink too but let me test it on my scrap vachetta first.
Now you have me curious! Hahaha! I'll try it first. I don't want you to mess up your wallet! I'll take pics and show you sometime today!