Item 21 is not a 'bill or invoice'. It's a questionnaire (Fragebogen) for reporting a person's income from various sources. Below the (blank) lines for name and address, it states: "To the Personnel Section of the Reich (national) Party Directorate of the NSDAP".
"I declare my income to be as shown below, and affirm that my responses are made to the best of my knowledge and in good conscience."
"I receive per month:"
On line a, "Pension" is crossed out, and in pencil and then pen is added: salary: 1,342.61 Reichsmark
Line g, "Per Diem" lists "Reichstag" (national parliament) 540.-- Reichmark
Total monthly income:  1,882.61 Reichsmark

The date line reads "Würzburg, the 19th of September 1937."