Not sure I understand all the details here, but as I read your posts, it sounds as if you and your employer initially agreed that you would be paid 2800 CHF for four ten-hour days of work, with Mondays and weekends off.

If that is the case, it appears to me that your employer merely changed the percentage on the permit paperwork from 80% to 100% based upon the comments of the gemeinde personnel as they saw that your actual hours per week would be 40, not 32.

Without adding any comment on whether your pay rate is fair or not, I will say that I don't understand how the written change for purposes of the permit paperwork translates to your employer being obligated to pay you more, if indeed you initially agreed to perform the forty hours weekly for 2800 CHF.