Tortious interference I would guess. Plaintiff wants to sell a "Doig painting", but defendant is preventing the sale by making a "false claim". So basically he'd owe damages for the lost sale proceeds.
This is so ass backwards. With this one could try to sell anything fake as part of someones works and if pointed to be wrong just sue for damages. Imo this should be treated as forgery and the guy selling the painting should be punished.
Once you've won a court case proving it isn't a forgery