Every contract is legally binding in Germany, even verbal contracts or in this case price tags (to some degree). Obviously other laws may invalidate them and verbally is hard to prove. For example if you advertise onetime off prices for a week to lure people in the store you have to have a reasonable amount of these items to be available through the week, otherwise people are eligible to get the offer or compensation.
Adding your own sticker would probably be fraud and easy to prove for the store (not matching sticker, no plans to reduce prices, …).
Have you checked out Battlerite? Not sure if it still has an active player base or it’s working on Linux.