just a point if the gentleman purchased a car this would put a different light of expenses. I know that you would need around 5000zl a month to enjoy a average lifestyle
I agree. Having a car in Warsaw would significantly increase costs, and essentially eliminate the ability to save anything more than, perhaps, a small pittance. If that.
When I said frugal existence, I excluded having a car. That would be a luxury. However, it is quite possible to live comfortably in a large city like Warsaw without a car. Unless, perhaps, he is a salesman or something similar and has to visit multiple dispersed locations during the course of the workday.
Another thing I did not calculate in to the equation was wardrobe, on the assumption that this is a short-term assignment and that the OP already has appropriate business attire that he will bring with him. He will, of course, have to buy a winter coat, gloves and hat, and perhaps a sweater and more durable shoes, but this should set him back no more than 1000 PLN, or less. If, on the other hand, he has to buy appropriate business attire in Poland, that can cost quite a bit, and further reduce savings potential. Also assumed that the apartment he finds would be more or less furnished, and that the most he would have to buy is a used desk and chair, bedclothes, and basic kitchen and bathroom supplies. Also assumed he doesn't smoke, which will set him back about 400 PLN a month at a pack a day. Or drink or go out to bars and clubs, which can also be rather costly.
All in all, I agree that the OP will not be able to save a substantial amount of money, if any at all, and should reconsider accepting the assignment if saving up a substantial sum of money during his stay is a primary goal of his. That goal is unattainable, even without having a car, without having to buy appropriate business attire, and without having to furnish his apartment.
There's also a big difference between how much money a native Pole who knows the country and the language needs to survive at a comfortable level, and how much a clueless foreigner fresh off the plane does. It is possible for a native Pole to live a modest life with a modicum of comfort in Warsaw for 3000 PLN. It would be difficult for a foreign SAP consultant to do so without living a very frugal, monkish existence.