At least my conscience is clean. I checked out the store at about 12:20 and batteries were still available, €25 apiece. I gave it a little thought and decided,

1. My Jolla does pretty well with its original battery;
2. So does my N900, with a battery it came with (I had it for at least 2 years and it was already used);
3. In the worst case, I am sure I can improvise something...
4. ...much cheaper than €25

So, someone else got the battery that I could have bought. I accept thanks on postcards, cash, cheques or PayPal