re: how can I justly avoid paying taxes on this?

The simplest way to avoid paying tax on all of the money you were paid is to have the payer issue you a 1099-Int for the portion of the money that was interest. You report that amount as interest income, and the balance of the payment is not reported as income as it is the repayment of the loan principal.

If there was no interest paid on the loan, then you need to follow the rules for below market rate loans. You'll have to look them up, or consult with someone familiar with them, to see what the requirements are since they depend on the facts and circumstances of the original transaction and any subsequent payments.