When a litigious right is assigned, the debtor may extinguish his obligation by paying to the assignee the price the assignee paid for the assignment, with interest from the time of the assignment.
A right is litigious, for that purpose, when it is contested in a suit already filed.
Nevertheless, the debtor may not thus extinguish his obligation when the assignment has been made to a co-owner of the assigned right, or to a possessor of the thing subject to the litigious right.