I have more questions than answers, but can see one issue right away--unavailability of 30 year terms for your smaller loans. Look at your typical deals and see whether this practice could impact a significant percentage of your applicants.
Using a simple interest rate of 3%, a $100,000 loan would have 360 pmts of $421.60. After applying your $500 minimum payment rule, the loan could not have a term longer than 278 months. For lower IRs or loan amounts, the maximum term is even shorter.
At an IR of 3%, the break-even point is $118,595. What percentage of your new loans have principal amounts less than $118,595? All of them would have been disadvantaged by a $500 minimum payment policy. Now, look at the profile of that disadvantaged group--is it skewed in the direction of any of the protected characteristics? If yes, there's a disparate impact case to be considered.
Now, add another factor--secondary market requirements. Do your investors purchase loans with odd maturities, or only 20, 25, 30 year am schedules? If there's any restriction in your placement options, will this work to the disadvantage of the applicants subject to the minimum payment rule?
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...gone fishing.