A Banksy artwork that appeared on a nondescript garage in a Welsh town has been sold to a dealer for a six-figure sum.

The garage’s 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.

Lewis, a steelworker, who has found it hard to sleep after unexpectedly becoming the owner of a precious piece of art, is said to be hugely relieved.

An estimated 20,000 people are thought to have visited his garage since it appeared just before Christmas and round-the-clock security has had to be introduced to protect the artwork. It created a cottage industry in souvenirs – mugs, coasters, keyrings – carrying the image of the work.

Ian Lewis standing in front of the work, painted on his garage. Photograph: Adam Hale/PA

Brandler said Season’s Greetings would stay in Port Talbot for a minimum of two to three years but might be moved into the town centre. He said Lewis had turned down higher offers to keep the work in the town.

The Banksy appeared just before Christmas in Taibach, close to the Tata steelworks. From one angle, it shows a child in a bobble hat with a sled, apparently enjoying a snow shower and trying to catch the flakes on their tongue. But from another, it becomes clear that what is falling on the child is a shower of ash.

Banksy confirmed the work was his by releasing a video of the mural accompanied by the Christmas song Little Snowflake. The camera rises above the garage and shows Port Talbot’s rooftops and the billowing chimneys of the steelworks and other industrial buildings.

Lewis was amazed when visitors began arriving in their droves and realised he would have to find a way of protecting the piece. A local operatic company paid for it to be shielded in plastic and the actor Michael Sheen, who went to college in Port Talbot, stepped in to pay for security.

Lewis said: “The offer was less than others but this man agreed to keep it in the town. Other people wanted to take it away and bring it back from time to time. I didn’t fancy that. I’m glad to have sold it – to be honest I’m fed up with it. It has been a strain.”

He plans to spend the money on a “few holidays”, and to replace the garage at the back of his home.

Lewis said: “I think the town has fallen in love with it, I think everybody loves it and it would have been a shame for it to move from the area. There were lots of offers, lots of options of what people wanted to do with it. There were art collectors, dealers, people who wanted to take it away basically.

“The artwork is going to be moved and I’ll rebuild the garage. I think that every graffiti artist in the area is going to come down and have a go at it now though, although I don’t think Banksy will be back again.

“I’m still thinking about what to do. Now the stress has been lifted off me I should have more time to think about what I really want to do.”

Brandler bought the Banksy before seeing it in the flesh but said he believed it was one of the artist’s best. “I’m so pleased to have this piece,” he said. “It’s not one of his most valuable but I think it’s one of his most powerful. It’s so 21st century, so poignant. I like his punchy pieces best and this is one of them.”

Brandler spoke to the Guardian from a train as he travelled to Port Talbot to see the Banksy in the flesh for the first time. He will be speaking to Lewis and Welsh government and local authority officials to decide what the next steps are.

“I want it to stay in Port Talbot for a minimum of two to three years,” he said. A strong possibility is that it will be moved from the garage site to a local shopping centre.

Brandler said he hoped that other artwork, such as other Banksys and pieces by Damien Hirst, could be brought to the town to show alongside it and young people could be taught how to create work like Banksy. He has also been in talks with a charity that trains homeless people with a potential plan to open a cafe near to the work’s new home.

Brandler is keen that it will remain free to view the work. Where the piece will ultimately rest is unknown. “Who knows?” said Brandler.

Of Lewis, Brandler said: “It’s been a huge pressure on him. He is very glad it has been resolved.”

Bethan Sayed, the Plaid Cymru South Wales West assembly member, said: “The new agreement seems to suggest that it may only stay in the town for a number of years, but hopefully we can discuss a longer-term plan with the new owner.

“I like the idea of bringing more Banksys to Port Talbot, but this should be in line with developing street artists in the town, and in Wales in general. How about a street art festival for Wales in Port Talbot? I wish Ian Lewis all the best for the future, and thank all those who have been involved. It’s vital now that the Welsh government works with the new owner to maximise this potential.”