I was reading this answer to a recent question about a planet traveling at the speed of light, which contained the lines:

Since time is money, this could be exploited for various economic effects. The simple example of a person collecting compound interest in the outside galaxy while residing on the planet shows some of what is possible, and eventually many more subtle and complex financial instruments could be devised based on the time differential between the planet and the remainder of the galaxy.

In this hypothetical universe, civilisations have the capability to feasibly travel to various parts of the galaxy where they would experience significant time dilation (e.g. planets traveling at the speed of light, close to the central black hole, etc) and safely return home.

If banks/financial institutions were aware of this, what sort of "complex financial instruments" would they devise to prevent situations like the one in the quote from destroying their economies? I am chiefly interested in what would replace the concept of "interest", since if time is cheap/almost free it seems rather meaningless to earn money for essentially nothing.

(Edit: removed speculation about currency zones as distracting to the main question)