mortgage companies offering payoff at discount

[Paul M.]

I remember hearing stories from my finance professors about how when interest rates were high in 70s the banks would offer mortgage borrowers a chance to payoff their loan at a discount to the principal balance.   Presumably the discount offered was less than the delta between bank's cost of capital and the mortgage interest rate.   With all the consumers that have mortgages in the 2-3% rate, how high would mortgages need to be before mortgage companies want to get rid of them?    Purely theoretical fantasy at this point, though it did happen in my lifetime (I was a toddler at the time).