Is it permissible to buy and sell copper pipes of varying lengths and types, with differing prices, if they are to be used for connecting the house to city gas?

The quantity of copper that the person bought from you is now their property. Their return of two meters of it and taking a replacement from you is considered a new, permissible sale, as long as the conditions of sale are met, namely mutual consent and knowledge of the item being sold, even if there is a difference in value between what was returned and what was taken.