Derek McLennan discovered the 10th century relics – including silver bracelets, a gold ring, an enamelled cross and a bird-shaped gold pin – under a Dumfriesshire beach in 2014.
The 52-year-old metal detectorist sold the trove to National Museums Scotland for £1.98 million three years later, making it the most valuable Viking haul ever discovered in the UK
But now kirk chiefs are demanding he hand over half that amount because his find happened on their property.
Rules on discoveries in Scotland mean only the finder receives payment, differing from the rest of the UK where awards are split with the land owner.
It is said Mr McLennan, a retired businessman from Ayr, made a written agreement at the time to give the church half the value of his bounty.
Church trustees are taking legal action at the Court of Session in Edinburgh because they say he has failed to honour that promise.
“The church has been unable to get a hold of him," a source told The Sun. "It doesn’t seem like there’s anything that would preclude him from being in touch. It appears to be a choice.It’s a tricky one for the church chasing the money but they do feel that they do have an entitlement of some kind of fee.”
A Church of Scotland spokesman said: “It can be confirmed the general trustees of the Church of Scotland have raised an action against Derek McLennan. As that is now a matter before the court it would be inappropriate for us to provide any further commentary at this time.”
Mr McLennan has been contacted for comment.