The housing association says it was a mistake within a formal deed transferring the ownership of the manse's one-bedroom ground-floor flat to a new buyer earlier this year.
It is believed the sale went through for £65,000. ACHA is now taking steps to rectify the position.
A spokesperson for ACHA said: 'There was an error within the Disposition in that the plan attached to it included an area of land not comprising part of the property being sold.
'The association’s position is that the transfer of land adjacent to Free Church Manse was made in error and was not part of the agreed sale contract. The association are taking steps to rectify the position.'
Two ACHA properties built in 2017 are on the land affected, but the housing association says the transfer does not relate to them.
Those two- and three-bedroom family homes, which added to ACHA's other nine properties on the island from the 1960s, were built with funding from Scottish Government, Argyll and Bute Council and ACHA itself to an overall project value of £505,000.
Original estate agent details selling 1A Free Church Manse described it as sitting 'in a delightful island location within the village of Arinagour. The property had been empty for some time and was showing signs of neglect it said advising potential buyers to study the housing report before arranging viewings'.
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