After their release, the managers resumed talks with workers at local government offices.
"We hope that this time our voices will be heard," Patrick Hachaguer, a union worker, was reported by the Reuters new agency as saying.
The Sony plant, which produced magnetic components and employs 311 workers, is scheduled to close for good on April 17.
Foucher was taken hostage during a visit to the plant on Thursday, his last visit due before its planned closure.
Sony had considered converting the plant to make solar panels, but abandoned the plan, angering the workers who had hoped to keep their jobs.
Workers' tempers have been boiling over in France, which, like other countries around the world, has been hit by a wave of factory closures and mass redundancies because of the global economic downturn.
On Thursday, workers in Clairoix, a small town in the north of the country, hurled eggs and shouted insults at managers in protest against the closure of their tyre plant by German car parts group Continental that would eliminate 1,120 jobs.