Short answer. Yes. Article 1807 requires every partner to account to the partnership for any benefit and hold as trustee any profit derived, without the other partners' consent, from a transaction connected with the partnership or from using its property. He must turn the profit over rather than keep it for himself.
What the law says
Every partner must account to the partnership for any benefit, and hold as trustee for it any profits derived by him without the consent of the other partners from any transaction connected with the formation, conduct, or liquidation of the partnership or from any use by him of its property.
Civil Code, Article 1807 — Partner as Trustee of Benefits. Read the full provision →
The partner holds the profit as trustee, not as owner
Article 1807 imposes a direct fiduciary duty: every partner must account to the partnership for any benefit, and hold as trustee for it any profits derived by him without the consent of the other partners from any transaction connected with the formation, conduct, or liquidation of the partnership or from any use by him of its property. Once a profit falls within this description, the partner does not own it outright; he holds it in trust for the partnership. That framing matters, because a trustee cannot simply keep trust property, and the partnership is entitled to demand it back as the beneficial owner.
Why consent is the line between a legitimate gain and a breach
The obligation to account and hold as trustee applies specifically to profits earned without the consent of the other partners. If the other partners had actually agreed to let this partner personally profit from that particular transaction or use of partnership property, the situation would fall outside this rule. But where, as here, the profit was earned secretly and without your consent, the absence of that consent is exactly what triggers the duty to account and surrender the benefit rather than keep it.
What kinds of profit this covers
The article reaches profit from three connected sources: a transaction tied to forming the partnership, a transaction tied to conducting its ongoing business, a transaction tied to liquidating it, or simply from using partnership property. Using partnership property to generate a personal profit falls squarely within the last category, so it does not matter that the underlying transaction might otherwise look unrelated to the partnership's regular business; using its property without consent to make money is itself what brings the profit under this duty.
What the partnership can do about it
Because the profit is held in trust, the partnership can demand that the partner account for it and turn it over, treating it as partnership property rather than the individual partner's personal gain. This duty exists independently of whatever other consequences the partner's conduct might carry within the partnership, such as effects on management authority or the partners' relationship going forward. The core legal point is straightforward: profit made secretly from partnership dealings or partnership property belongs to the partnership, not to the partner who made it.