Informing the customer about a defect in the product

I sell 1m lengths of pipe at R80. I purchase these ready cut. I have a marking on my counter to measure 1m. Those who purchase don't specifically require 1m but purchase the metre as that's how it's sold. It is advertised as 1m. I noticed a pipe today that was marginally short. I therefore refunded R2 to the customer but did not inform him why. The pipe will still fulfill the function for which it was purchased. What is the ruling regarding this transaction?