A renowned American artist has been accused of plagiarising and profiteering from an iconic image by a South African photographer that came to symbolise the collapse of apartheid.

Graeme Williams said he was astonished to walk into the Johannesburg art fair last week and see his photograph of a group of black schoolchildren taunting despondent, armed white policemen sitting on an armoured car shortly after Nelson Mandela’s release in 1990. The image was reproduced with relatively little change, except that it was drained of colour and lightened in parts, under the name of Hank Willis Thomas.

Williams was even more astounded to discover the asking price of $36,000 – 25 times the amount he has ever earned from the photograph.

Photo taken at the Johannesburg art fair by Graeme Williams of Hank Willis Thomas’s version of Williams’s colour original. Photograph: Graeme Williams

Thomas is known for artwork that takes advertising, strips away the sales pitch and leaves the remaining images to speak for themselves. His work also includes a permanent installation at the National Memorial for Peace and Justice in Montgomery, Alabama, depicting black people emerging from the top of a wall with their hands raised in surrender in a comment on police violence.

“The changes were absolutely minimal. It’s theft, plagiarism, appropriation. It’s a kind of fine line where you say it falls,” said Williams. “Within the art world there’s an acceptance that you can use images within the artistic framework to create something that has meaning different to the original image. This was the exact same of my original photograph and all he had done is take an image that he likes and call it his own.”

The photograph was taken in Thokoza township south of Johannesburg as Nelson Mandela arrived for a rally weeks after his release from prison. The image was heralded for capturing the sudden shift in power away from the apartheid system and has regularly appeared in exhibitions around the world.

Thomas’s use of the picture removes the colour and lightens the police officers so that they can be viewed clearly only when standing directly in front of the work. Williams said he did not regard this as amounting to a new piece of art created out of the photograph.

Thomas defended his use of the image in part by questioning whether Williams can lay claim to ownership.

“I can empathise with his concern and frustration but there are critical questions about who has the right to the image and whether it be subjects of the image, who I am most interested in. If the subjects of the image were compensated or remunerated. If they were asked. There’s a lot of questions related to representation, objectification, exploitation,” he said.

“If a photograph is 25 years old, 40, 100 years old, of public events where most people who are in the photograph are not given control over how they’re depicted and what’s happening, when is it ours as a society to wrestle with?” he asked. “I think of it as more akin to sampling, remixing, which is also an area that a lot of people said for a long time that rap music wasn’t music because it sampled.”

Williams is a highly respected photographer whose work includes an iconic photograph of Nelson and Winnie Mandela hand-in-hand with their fists raised as the former political prisoner emerged from jail. Williams has exhibited at the V&A in London and the Smithsonian in Washington DC. He has also been published in the Guardian.

The use of photographers’ work by artists has become an increasingly fraught issue given the ease with which the original pictures can be accessed on the internet. Several photographers have sued the artist Richard Prince for copyright infringement over his use of their pictures in collages and paintings. A New York court ruled last year that at least one of the cases can go ahead because Prince’s work was not transformative enough of the images to amount to original art.

Thomas’s work was on display as part of the art fair at the Goodman Gallery in Johannesburg. The gallery’s owner, Liza Essers, said she regards both men as having “incredible integrity”. She said she was not in a position to judge whether Thomas’s piece was transformative of the original photograph but noted that he has long used his art to question the ownership of images.

“That has been fundamental in his work since the beginning of his career. We’ve worked with him for nine years and he has an ongoing interrogation and questioning of the ownership of images,” she said. “Hank is questioning the ownership and authorship of documentary photography. The question of copyright or not is irrelevant to what Hank’s asking. They’re coming from completely different places of departure.”

Thomas said he was “shocked” that the asking price for the piece was $36,000 and that it was set by the gallery. Essers said Thomas was being modest and that the price reflected the value of his art.

“That’s seen as a unique work by him,” she said.

Williams scoffs at the idea that the artwork is unique, and at the price. He said he has never sold the image at the centre of the dispute for more than about $1,200.

“The main theme of a lot of Hank’s work is to show the oppression of the oppressed. In this case he’s taken my photograph and added half a million rand, which is the cost of a medium-cost house in South Africa. It does feel like a mismatch between what he says he’s doing and what he’s really doing,” he said. “I wonder if he would have the same magnanimous attitude towards his own images within the public domain.”

The disputed work was removed from display after Williams complained to the gallery. Thomas has agreed not to show it publicly. He said that “Graeme’s feelings matter to me” and that he called Williams on Monday to offer to let him keep the piece for a year.

“I asked him to take the work, look at it, live with it, and then have a conversation with me about it in a year because I think the reaction is a kneejerk reaction. I think it’s not necessarily unwarranted but I think it’s a reaction that requires a deeper conversation, which the work is really about,” he said. “If he chooses to he can destroy it. He said he did not want to destroy it, that he thought we should destroy it. I told him I wouldn’t do that because if I thought I was doing something wrong I wouldn’t have made it.”

Williams was not impressed by this idea.

“It’s utterly bizarre. I take this artwork and keep it for a year and then we’ll chat after that? Fuck knows what that means. Perhaps that after a year I’ll understand the complexity of his artwork. I said no thanks. I have my own version of the photograph and I really don’t need an American to give that image some kind of significance and meaning,” he said.