Valls, who was due to visit a former slave-trading post in the Ghanaian capital, Accra, on Sunday, published his article in France's Le Monde daily and in the English-language Africa Report.
Slavery was a "disaster on a grand scale", he wrote, a "crime against humanity" as "France has fully acknowledged" in a law passed in 2001.
But "memory should not divide. It should, on the contrary, close fractures and bring people together," he went on, citing black anti-colonial writers Frantz Fanon and Aimé Césaire to argue that slavery was "irreparable" and thus rule out financial reparations by the West.
Racism, anti-Semitism and 'competition'
In an apparent reference to those, like controversial French comedian Dieudonné, who claim that the Nazis' anti-Semitic holocaust is played up at the expense of recognising the crime of slavery, Valls condemned "competing memories, hierarchies and comparing the suffering of some with the misfortune of others".
"A memory that is shared and at peace arms us against racism, anti-Semitism, anti-Muslim and anti-Christian acts, xenophobia and hatred of the other, against prejudice and discrimination – all those abject forms of intolerance that continue to imprison and poison our societies because they represent as many insults, humiliations and wounds," Valls said.
The prime minister proposed a Euro-African version of the Europe's Erasmus students' exchange programme, as well as support and encouragement for young African entrepreneurs.
Hollande and debt to Haiti
When opening an anti-slavery museum in the West Indies in 2015, President François Hollande failed to respond to anti-racist campaigners' call for reparations but caused confusion by saying it was necessary to "settle the debt we have" towards Haiti.
Speculation that this was a reference to the 90 million gold francs (17 billion euros today) paid in compensation for income lost by French slave owners and settlers after Haiti became independent in 1804 soon proved unjustified.