The artwork in Port Talbot shows a child playing in falling ash from a dumpster fire, an apparent reference to the steel town's air pollution.
John Brandler of Brandler Galleries in eastern England said Friday he has agreed that the mural will stay in place for at least two years. It created a cottage industry in souvenirs - mugs, coasters, keyrings - carrying the image of the work.
Security guards were hired to protect the work after it appeared in December on a steelworker's garage.
Mr Brandler, from Brentwood, said: 'I'm so chuffed to have it, you have no idea.
'It's not just a girl holding a balloon or a bunch of flowers or something.
The garage owner, Ian Lewis, has sold the piece, Seasons Greetings, to the Essex-based Banksy expert John Brandler but it will stay in Port Talbot, at least for the time being.
'Ian was getting offers from people who wanted to take it away and not work with the town.
"I want to make Port Talbot a go-to place rather than a go-from place", he said.
"If we all work together I think we will get tonnes of people coming to Port Talbot".
Hollywood actor Michael Sheen has been contributing towards security costs to protect the artwork in his hometown, which included a donation to a protective plastic screen now covering the artwork. The British graffiti artist who keeps his identity a secret, confirmed yesterday that the artwork was his using his verified Instagram account to announce "Season's greetings" with a video of the artwork in Port Talbot.
At the end of the footage, which is soundtracked by Christmas music, the video pans up to show the industrial landscape of Port Talbot steelworks, which is thought to have inspired the painting.
The artwork appears months after Port Talbot was mistakenly dubbed the most polluted town in the UK.