I have a serious problem.

I went today into a Volkbank office in Hombruch (Dortmund district). I had 740€ with me and got only 640€ back! I noticed this
missing 100€ only at home at evening.
It was a serie of confusion and rush. I had changed some money in two other banks nearby so I wasn't completely aware how much money I had at the moment (it's difficult when you have huge amount of different notes with you). The cashier counted the money only
by hand (didn't use the counting maschine right next to her) and asked me after that how much there should have been. I was confused by the rush and I just said 640€ and she said "ok" (I thought she has counted the same amount and everything was right). Then she handled me 640€ in fivers. I didn't think about it anymore and left. Later at home, after entering the notes I began to think about it when my calculations didn't match. I had 100€ too few!
Now I'm angry and desperate.
Is there anything to do for it?! In Finland I know they count the money at cashier after every day so they know if there's money missing (or too much). Do they so also in here?! In Finland I wouldn't be this worried, because they know me and would probably understand this, but in here they don't know me (I hope the same woman will be there in the morning and remember "the guy changing awful lot of fivers") and I don't get any receipt from the exchange, since it's not done via my Volksbank account (I have one), but just "in-the-air" changing notes.
Please tell me they will notice the extra 100€ themselves and tomorrow when I go back there they will remember me and give me back my 100€. Or if they refuse to do so, what then? Is there anything I can do? 100€ is awful lot of money for a student like me and this mistake (it was both my and the cashier's mistake, by not paying enough attention to the situation) is really making me feel bad.
