Suppose an island where the only food is fresh fish. Crusoe has, say, 10 fish, and he knows that in a few months he won't be able do catch them by himself (he's too old). He then decides do exchange with Friday, and the deal he gets turns out to be 10 present fish for 9 future fish.
I thought in two answers for the negative spread: fish when able to work and fish when unable to work are different goods (like the commom "ice objection" to Time Preference), or that there must be something like a purchasing power premium for the rottening of the fish (it's value is not transported through time)...
Any thoughts?
TP applies to homogeneous goods. Your first answer is IMO correct, you're dealing with heterogeneous goods.
Freedom of markets is positively correlated with the degree of evolution in any society...