Court documents seen by AFP Friday show that the "Sultan of skinny", famed for his skinny jeans look, was only paid 667,000 euros of the 10 million he had been promised for his last year in charge of French label Saint Laurent.
A spokesperson for Kering told AFP the group was appealing the decision.
Slimane, 49, one of fashion's hottest and most enigmatic talents, went through a messy divorce with Kering, the brand's owners, in 2016.
He has since moved to the rival luxury conglomerate, LVMH, to take the reins at Celine, where he will also create a men's line in a massive overhaul of the brand.
The latest payout comes two years after a Paris court ordered Kering to pay Slimane $13 million concerning a non-competition clause in his contract. The group also appealed that ruling.
Slimane's skinny and rock star chic looks made millions for both Dior and Saint Laurent, and were much copied by mass-market brands.
Chanel boss Karl Lagerfeld once famously shed 41 kilos (90 pounds) in order to squeeze into Slimane's skinny jeans.
Like Lagerfeld, Slimane is a renowned photographer, and he has spent the last seven years living in Los Angeles, where he had moved his studio at the end of his reign at Saint Laurent.
The designer drew much of his inspiration from the LA rock scene, which he tirelessly documented with his photographs and in his blog.
A mysterious and intensely private figure, he nonetheless has struck up close friendships with many A-listers including pop star Lady Gaga.
The Libertines' bohemian frontman Peter Doherty also became a friend and muse, and figured prominently in his 2006 photo book, "London Birth of a Cult".
Born into an immigrant family in Paris, Slimane's father was a Tunisian boxer who became an accountant and his mother an Italian dressmaker.
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