A PACT between the DUP and the Conservative Party could be challenged in the courts.
Mental health worker and Green Party member Ciaran McClean has launched a crowdfunding campaign to take a judicial review against the deal.
The DUP has agreed to vote with British Prime Minister Theresa May's minority Tory government in key debates in return for an extra £1 billion for the north.
However, Mr McClean said the deal is "unlawful" and in breach of the British government's requirement to be an impartial broker between the north's parties under the terms of the Good Friday Agreement.
Cartoonist Ian Knox on the threat of legal action over the Tory/DUP pact
"I wish to hold the Government to account for their actions through the courts in a judicial review," he said.
"My claim is that as a citizen I expect my government to honour its obligations under the Good Friday Agreement and not to bribe others with money so that it can stay in power."
He aims to raise £20,000 to take the judicial review. His campaign has so far raised more than £17,000 on the crowdjustice.com website.
Meanwhile, an anti-DUP banner was flown during Saturday's annual Pride parade in London in protest at the party's stance on same-sex marriage.
The banner, saying f*** the DUP, was attached to balloons and flown at Nelson's Column in Trafalgar Square.
Unlike the Republic or Britain, same-sex marriage remains outlawed in the north.
The DUP has used the controversial petition of concern mechanism to prevent a change in the law, despite a majority of MLAs supporting the move at the last vote.
Around a million people took part in the Pride parade, which marked the 50th anniversary of the part-decriminalisation of homosexuality in Britain.
Same-sex sexual activity has only been legal in the north since 1982.
