The National Portrait Gallery has launched a campaign to stop the export of the earliest known British oil painting of a freed slave.
It needs to raise £100,000 to save the 1733 portrait of Gambian Ayuba Suleiman Diallo, by artist William Hoare.
The Heritage Lottery Fund, The Art Fund and the NPG will all contribute to the total cost, which is £555,000.
The portrait, which has never been seen in public, has temporarily gone on show at the London gallery.
Diallo, who was from a prosperous family of religious clerics, was captured as a slave in Africa at the age of 29 and was transported to work on a plantation in America.
After he was imprisoned for trying to escape, he met lawyer Thomas Bluett who, impressed by Diallo's intellect and education, took him to England.
Diallo was received with great enthusiasm by aristocrats and scholars.
The National Portrait Gallery said Hoare's painting was the first British portrait that honoured a named African subject as an individual and an equal.
A spokesman for the gallery told the BBC News website that the work - which had been in a private collection - had sold at auction in Christie's in London in December for £554,937.50.
Following the sale, then Culture Minister Margaret Hodge put a temporary export bar on the painting and gave British museums and galleries the opportunity to buy it if they could match the price.
The Heritage Lottery Fund and The Art Fund have already given grants of £333,000 and £100,000, while the NPG has also contributed.
The Lottery Fund's grant will go towards the acquisition, as well as the costs of conserving, displaying and touring the painting around other galleries and museums in the UK.
Broadcaster and National Portrait Gallery trustee, Zeinab Badawi, said: "This portrait is a rare example of a painting of an 18th Century African in Britain.
"This portrait would be a vital and powerful addition to the collection at the National Portrait Gallery for its representation of Britain's diverse cultural heritage."