“Highland Council’s rush to monetise a common good asset has completely broken the system,” a spokesperson for the museum said.
“The council has created a nightmare scenario where the community is punished for the council’s own unethical sales process.”
The spokesperson added: “We are expected to raise more than £3m to buy back our own property, while being barred from the very grant funds built to preserve national treasures.”
Trustees are urging Scottish ministers to intervene and have launched an online crowdfunding campaign.
Highland Council said it had followed all relevant legal, governance and decision-making processes.
It said it had carried out a statutory public consultation, adding that “a majority of respondents” had been in favour of the sale.
The council added that it had made an application to a sheriff court seeking authorisation to sell the bust.
It said no objections were received during that process and the sheriff approved the proposed sale.