The value of the loan had it not been discounted is easily established through the expert testimony of an economist. In fact it would be required of the plaintiff in order to win the case. The Plaintiff would claim the difference between the actual sale price of the mortgage and the market price as established by the testimony of the economist.

I had a similar case years ago in which a developer hired an an engineer to subdivide property. There was a dispute that arose that delayed the subdivision. The developer claimed that the sale of the property was diminished by the delay because the real estate market cooled before the property could be sold. We were able to get an economist from the University of Connecticut to reconstruct the market price had the sale taken place as planned. The developer claimed damages created by his loss of his window of opportunity

The court usually favors settlement of claims, and would encourage the parties to negotiate among themselves. As to what the plaintiff can live with depends on the negotiation.