Since Germany was mentioned specifically, I'm curious what the laws are around decreasing pay, if it's as hard as firing someone, if some threshold or percentag

Since Germany was mentioned specifically, I'm curious what the laws are around decreasing pay, if it's as hard as firing someone, if some threshold or percentage would be considered a constructive firing, etc. This could eliminate some of the risk of high compensation if, for example, you could hire someone at $100k/yr USD equivalent and then after a reasonable review cycle lower the pay for bad performance.