- In the Japanese card, there is a text saying "If Balloon Berry is attached to one of your Pokémon, the Pokémon.'s retreat cost is 0". So, you should play Balloon Berry as if it negates the Pokémon's retreat cost while attached to that Pokémon, not just when it actually attempts to retreat. For example, Muk-EX's "Slimy Water" attack damage will be reduced to only 40 against a Pokémon with Balloon Berry attached to it.
Source:
PUI Rules Team (2003-11-13)