Geschäft ist Geschäft. — Business is business. Duden explains it: when money is involved, one cannot take feelings into account.
Beim Geld hört die Freundschaft auf. — Where money begins, friendship ends.
Bei Geldfragen hört die Gemütlichkeit auf. — Where money is concerned, the friendly mood ends. The banker David Hansemann said it in the Prussian United Diet in 1847, in a debate on a state loan for a railroad. It quickly became a common saying.
Dienst ist Dienst und Schnaps ist Schnaps. — Duty is duty and schnapps is schnapps.