Piggy Bank is a special offer that returns a part of the emeralds you spend. The offer unlocks at team level 20. It appears on a set schedule and is available for two days.
You can find the offer on the right side of the main screen among other offers:
Every emerald you spend automatically counts toward filling your Piggy Bank. For every 5 emeralds spent, 1 emerald is added to the bank. When you spend emeralds, a pop-up shows your updated progress:
Only the first 50,000 emeralds you spend during the offer count toward the Piggy Bank. Any emeralds spent after reaching this limit will not add more emeralds to it.
You can also reach several spending milestones that grant bonus emeralds to your Piggy Bank. For example, spending 12,500 emeralds adds a bonus of 500 emeralds.
Note: Cashback accumulates in the Piggy Bank while the offer is active. Once the timer runs out or the Piggy Bank is full, you can purchase the offer to break the Piggy Bank and claim all the accumulated emeralds.