Most post-docs are 100% jobs, but less-than-100% positions are also not unheard of. You may find that this percentage changes (i.e. increases) if more funding becomes available, either through collaborations with another lab/research group that will pay for part of your salary, or through grant monies that come in. It's worth asking your future supervisor about the possibility of your job percentage (and hence your salary) changing.

On the bright side, working 75%, you'll have some time off each week to get out and explore this beautiful country... and you can even get some shopping done during the day! (Which is a luxury...)

HeatherM