Why did he show the thirty kroner to the peddler?

When crofter was telling about her cow peddler proudly that his cow gave him enough milk and supports him financially. He told peddler that he had got thirty kroner last month as a payment for supplying milk to the creamery. But still, peddler seems doubtful about it. So, in order to assure peddler, he showed thirty kroner to the peddler.