Imbula joined Marseille earlier in the month and Labrune says the player arrived for the challenge.
Speaking to L'Equipe, Labrune said: "We paid 7million Euros without add-ons for Imbula. Chelsea, Paris Saint-Germain and Monaco were keen to pay almost double.
"We would not have signed him without the willing of the player to join us. Players come here for the challenge. The wages do not lead them."