Assuming without deciding that the Supreme Court holding that public sector labor unions are not able to compel representation fee payments from nonmembers is retroactive, a claim to recoup these funds under S1983 is an action for damages, not disgorgement, as the funds are no longer traceable.
Private citizens who rely in good faith on a state law later found to be unconstitutional can interpose a defense of good faith to an action for damages under the federal statute.
Assuming without deciding that the common law tort defenses at the time of the adoption of the federal statute determine which defenses are available to the statutory tort claim, the closest analogue to a private citizen's reliance on an unconstitutional state law is abuse of process, not conversion.