Wer zahlt, schafft an. Who pays gives the orders.
Duden lists the saying under the verb anschaffen. In standard German the verb means to buy something that lasts, such as a car or furniture. In southern Germany and Austria it also means to order or to command. Duden explains the saying: wer die Kosten zu tragen hat, darf auch bestimmen, the one who has to bear the costs may also decide.
The dictionary LEO gives the same idea in standard German. Wer bezahlt, darf auch bestimmen. Its English equivalent is: He who pays the piper calls the tune.