We can turn this on its head and ask a similarly tricky question: Should a human that can live for 1000 years be allowed to access interest rates that 100 year humans get?
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.