Sack

A poor farmer went to a market to sell some peas and lentils, however as he had only one sack and didn't want to mix peas and lentils, he poured in the peas at first, bound the sack up and than poured in the lentils. At the market a rich innkeeper wanted to buy the peas, but he did not want the lentils.
How would you solve this problem if you had only the sack of the innkeeper, which he wants to keep (without devaluing the goods).

Solution:
Pour the lentils into the innkeeper’s sack, bind it and turn inside out. Pour in the peas. Then unbind the sack a pour the lentils back to your sack.