Actually Poland is one of the biggest “payers” in EU. The money payed is being calculated according to GDP, population size etc.
Additionally Poland has a very strong economy, being the only EU country which economy did not suffer from the global breakdown, keeping constant growth.
My point is that they can’t simply suffocate polish economy, as they would feel the fallout in theirs as well. And Germany can’t afford it right now. Especially after Portugal and Greece went basically bankrupt.
Also they now have a lot of expenses thanks to the “immigrants”. Welfare mostly.
And deep down Germans want Poland to be the “bug out” place for the Volksdeutsche, once the Muslims make Germany intolerable for them.