From wiki article on Philip II of France in the section 'Battle of Bouvines, 1214'. Full paragraph given for context.
Basically, Count of Flanders refused to join Philip in his war against King John of England, and so Philip was given papal permission to punish Flanders (ironically, after the papacy had said it would be a sin to declare war on King John).
"The French fleet, reportedly numbering some 1,700 ships,[25] proceeded first to Gravelines and then to the port of Dam. Meanwhile, the army marched by Cassel, Ypres, and Bruges before laying siege to Ghent.[25] Hardly had the siege begun when Philip learned that the English fleet had captured a number of his ships at Dam and that the rest were so closely blockaded in its harbor that it was impossible for them to escape. After having obtained 30,000 marks as a ransom for the hostages he had taken from the Flemish cities he had captured, Philip quickly retraced his steps to reach Dam. It took him two days, arriving in time to relieve the French garrison, but he discovered he could not rescue his fleet. He ordered it to be burned to prevent it from falling into enemy hands, then he commanded the town of Dam to be burned to the ground as well. Determined to make the Flemish pay for his retreat, in every district he passed through he ordered that all towns be razed and burned, and that the peasantry be either killed or sold as slaves.[25]"
This is, unfortunately, only a partial answer, but it's very likely that he didn't, and that the Wikipedia article is wrong.
The only reference the Wikipedia author has used for his/her entire narrative of the Bouvines campaign is The History of France: From the Final Partition of the Empire of Charlemagne A.D. 843 to the Peace of Cambray A.D. 1529 by one Edward Smedely, published in 1836. If you're wondering why the author used this 180-year-old text, rather than, say, one of the many modern scholarly works published in the last 30 years on Philip and Bouvines, it's most likely because it's available free on the internet. Check it out yourself here, courtesy of the Haiti Trust. The section on the Battle of Bouvines is on pages 70-72 and is so close to what's written on Wikipedia I'd probably get my ass handed to me for plagiarism if I turned that in as a college assignment. The other great thing about this Smedely chap is that he, typically, doesn't cite his sources particularly consistently, so I'm having difficulty figuring out where he's getting the tidbit about slavery from. His main source for the battle (as in, the only real primary source he cites for the battle) is Matthew Paris; that is to say, Roger of Wendover's Flores Historiarum, since Matthew Paris just copied Rogers work for everything before the 1230s. It's made more likely that this is Smedely's source since other major accounts of the battle, like the account of Philip's chaplain Guillaume le Breton or that from Richer of Sens' chronicle, don't mention the naval engagements at all. But when we actually go and read what Roger wrote, and you can find an old but decent translation here at Archive.org (with the relevant passage at pages 271-3), we can see that Roger actually says nothing about Philip selling slaves during his retreat. All he says is:
On learning [of the fleet's destruction] retired in confusion from Flanders with great loss to himself and to his followers.
A very different tone from Smedely's account.
I'll qualify this answer by saying that it's not totally impossible that there's something here, but it's incredibly unlikely, especially given that, as you point out, there was no slave trade in France to which to see Flemish peasants in. I (just like the Wikipedia editor) don't have access at the moment to the modern literature on Bouvines, and it's possible that Bradbury or Duby do comment on this and provide a primary source to back it up; but I doubt it. I'll check and update on Monday once I'm back in the uni library, but for now this is looking like unsubstantiated fantasy on the part of a 19th century author.