If Robinson wouldn't lend out his clothing it would soon rot and not be usable anymore. But if he lends it out, he wouldn't be charged with the expenses to preserve the value of his clothes at the present level. This is the calculation that is being made up here.
"This obvious fact has been overlooked by every writer upon interest up to the present day, even by Proudhon." (Gesell, A Story of Robinson Crusoe)