I hate to say it, but you blew your chance. Sure, you'll be able to see her again to give her the void cheque, but if you ask her out when you see her again, she'll probably think: "hey, he conveniently forgot to give this to me so he could see me again to ask me out... how creepy."
Now, I'm not saying that you shouldn't ask her out anyway, because there's always a chance that she might think: "hey, he conveniently forgot to give this to me so he could see me again to ask me out... how sweet." The odds are against it, but you've got absolutely nothing to lose by trying, so give it a shot.
My advice to you is to not build it up as some big thing. Convince yourself that you don't care if she shoots you down. That way, there'll be no reason for you to be nervous at all when you ask her. If you're not nervous, you'll be more able to just be yourself - the confident version of yourself.
If you get rejected, not only is it no big deal, but it'll actually be good for you. You'll see that being shot down by one woman isn't the end of the world, and seeing that it's not so bad, you'll be more comfortable with asking out the next girl. The more comfortable and confident you are, the better your chances.