The Pension Pig is a consumable purchase, meaning you have to purchase it each time you open it.
The total bux stored in the Pension Pig increases with every city and game upgrade you make as well as for every bux you collect in the game.
When you make this purchase, you receive the accumulated bux that are shown at the time of purchase. After collecting, the bux in the Pension Pig are reset to zero and continue to increase as before. To open the Pension Pig again, you need to make the purchase again.
Using the Prestige feature does not reset the bux stored in the Pension Pig.