Given the state of the market at the moment, if I were in your shoes I'd negotiate with the developer as the last thing they want is to have to give any money back and be left with an apartment they can't sell.
Just following on from this suggestion, can you check to see if there are still apartments available in the development that you bought. And if Yes, can you see what prices they are now going for? There has definately been a decrease in prices since your initial purchase, and while the level of decrease may differ from city to city, I dont think it would be extreme to think that prices have gone down by at lease 5 % in Katowice. Try and check with some estate agents to see what has happened. Maybe then you can go back to the developer and re-negoatiate i.e. get out of paying the final 5%. Of course, this is totally dependent on your desire to keep the apartment in the first place.
If you are looking for the return on your investment, it will be an uphill struggle. There has been such a change in the property market over the past 2 years, that developers will be very unwilling to give back money especially if they have a lot of unsold stock on their hands.
Best of luck with whatever you decide.