This is a story i read in Garegin Fataee's blog and i translated it into English. i put it's link at the end of the text.)

There were a poor man whose wife made butter in round shape and each were 1 kg.He sold it to one of city grocers and he bought every thing they need in return.
One day the grocer doubt and he decided to weight it to find that whether it is 1kg or not.when he weighted ,it was 900 g .he became angry and in the next day he said to poor man : i would not buy your butter after this.you said it is 1 kg whereas it is 900g. the poor man saddened and said to him  head down :we don't have a scale to weight it. we bought you 1 kg sugar and we used it as a scale to weight.
Be Sure That We Measure  For You In Measure Of Yourself. Quoted by Fukaro

Link : http://www.garegin.blogfa.com/post-2178.aspx