The piggy bank is part of a special offer that gives you the chance to buy Township Cash at a great price, but you aren't required to take part. The special offer does not affect the rewards you get for regular in-game actions, nor does it take away any Township Cash that you could've gotten otherwise. You get the exact same amount of rewards that you usually do while the special offer is on. If you aren't going to buy Township Cash from the piggy bank, just ignore the offer altogether—you aren't losing any of your regular resources.