That will depend on temperature change between the two places.
I can show you the answer if we assume that the temperature doesn't
change, but that's so unlikely between 2 places where the pressure
changes by 50% that it's going to be a very unhelpful answer.
We also have to assume that the air in the balloon acts like an "ideal gas".
For an ideal gas at constant temperature, the product of
(pressure) x (volume)
is a constant number.
The balloon starts with pressure = 2.7 bar and volume = 8 L.
The product is 21.6 bar-liter.
If the temperature doesn't change, then that number won't change either.
The balloon is moved to a place where pressure = 1.3 bar.
Since the temperature doesn't change, we know that (press) x (vol)
is still 21.6 bar-liter.
So we can write . . .
(1.3 bar) x (volume) = 21.6 bar-liter
Divide each side by (1.3 bar) :
Volume = (21.6 bar-liter) / (1.3 bar) = 16.62 liters