Work pay is structured in five grades under section 52 StVG and depends on performance and duration of work.
Under section 51 StVG inmates are in principle obliged to work. Pay is determined under section 52 StVG in five grades. Grade 1 applies at the start of work; higher grades are reached step by step based on performance and time in service. The classification decision rests with the prison management on the basis of actual work performance.
What to do now: First, request in writing from the prison management your current pay grade and clarify the basis for its calculation. Second, check whether the grade matches your actual work performance. Third, if unclear consult a legal adviser who can review whether the grading is traceable.