In Gesell's story, unlike Fisher's, storing wealth requires considerable effort and ingenuity.
Meat must be cured.
Wheat must be covered and buried.
The buckskin that will clothe him in the future must be protected from moths with the stink-glands of a skunk.
Saving the fruits of Crusoe's labour entails considerable labour in its own right.
Even after this care and attention, Crusoe is doomed to earn a negative return on his saving.
Mildew contaminates his wheat.
Mice gnaw at his buckskin.
"Rust, decay, breakage...dry-rot, ants, keep up a never-ending attack" on his other assets.
Salvation for Crusoe arrives in the form of asimilarly shipwrecked "stranger".
The newcomer asks to borrow Crusoe's food, leather and equipment while he cultivates afarm of his own.
Once he is up and running, the stranger promises to repay Crusoe with freshly harvested grain and newly stitched clothing.
Crusoe realises that such a loan would serve as an unusually perfect preservative.
By lending his belongings, he can, in effect, transport them "without expense, labour, loss or vexation" into the future, thereby eluding "the thousand destructive forces of nature".
He is, ultimately, happy to pay the stranger for this valuable service, lending him ten sacks of grain now in return for eight at the end of the year.
That is a negative interest rate of -20%.
If the island had been full of such strangers, perhaps Crusoe could have driven a harder bargain, demanding a positive interest rate on his loan.
But in the parable, Crusoe is as dependent on the lone stranger, and his willingness to borrow and invest, as the stranger is on him.