Interest and Meta-Economy

If B returns to a satisfied A a dollar bill loaned a month previously, the interaction is equivalent to the earlier one.  However, if A has charged interest, then the value of even the selfsame bill is less than it had been, indicating both a depreciation of it over that period, as well as an inflating of the value of A's satisfaction, even if elsewhere, its purchasing power remains the same.  So, Interest can create an opaque Meta-Economy, that, nevertheless, usually influences the first-order Economy, e. g. determining a graduate to take a career path that can specifically accommodate the repayment of the interest portion of an education loan.  Nor do non-Marxist Economists seem to respect the distinction that the example illustrates, i. e. Marxists isolate what they could call a 'surplus-Economy', of which Interest is one factor.