Curious that you've framed the question in non-market terms (allowed by whom?).
> A 1,000 year human has many more opportunities
No more so than 10 generations of 100 year humans. People can and do coordinate over long time spans and multiple generations. Heck, that's basically what a corporation is.
> compounding interest will only grow his wealth further
Only insofar as they have deferred consumption for longer. This is how interest works.
> At the end of it, they have $358,816,... a huge amount of money.
Signifying the economic benefit from deferring consumption and instead putting those resources to productive use. This is a Good Thing.
> So any interest paid to the traveler should be calculated at the fraction of time experienced, so 5 minutes to 5 days would be 1 / 1440.
As you've described it, those travelers will already be more inclined to accept a lower interest rate than non-travelers. What's the problem?
> Near light travel time dilation is essentially life extension relative to planetary populations. So any interest paid to the traveler should be calculated at the fraction of time experienced, so 5 minutes to 5 days would be 1 / 1440.
Why should either the lender's clock or the borrower's clock be considered more valid for the purpose of calculating interest? Why is one privileged over the other?
A 1,000 year human has many more opportunities, and compounding interest will only grow his wealth further. Here's only an initial $1000 investment over 1000 years:
https://www.calculator.net/interest-calculator.html?cstartin...
At the end of it, they have $358,816,... a huge amount of money.
By year 730, they're quintillionaires. Then they have roughly 270 years to spend it, and on top of that, they can hand some of that wealth off to their offspring if inheritance rules don't change. (So really, it's the subsequent generations you really have to worry about)
Near light travel time dilation is essentially life extension relative to planetary populations. So any interest paid to the traveler should be calculated at the fraction of time experienced, so 5 minutes to 5 days would be 1 / 1440.
As for any opportunity cost, well, there really isn't any greater opportunity cost than being able to extend your life by that much and wait for interest to accrue. So by virtue of being able to travel at near light speed and dilate time, you're quite happy to forgo every other opportunity.