Short answer. Yes, from the day he used it. Under Article 1896, the agent owes interest on the sums he applied to his own use from the day he did so — and on the sums he still owes after the agency ends. So money your agent diverted to himself carries interest from the moment of diversion.
What the law says
The agent owes interest on the sums he has applied to his own use from the day on which he did so, and on those which he still owes after the extinguishment of the agency.
Civil Code, Article 1896 — Interest on Sums Applied to Own Use. Read the full provision →
Interest runs from the day of misuse
An agent handling the principal's money must keep it for the principal's purposes, not his own. Article 1896 attaches a specific consequence when he does not: the agent owes interest on the sums he has applied to his own use from the day on which he did so. Two points stand out. The agent owes interest — a return for the use of money that was not his to use. And it runs from the very day he applied the money to his own use, not from a later demand, a later reckoning or the end of the agency.
Why the early start matters
The choice of start date is deliberate and it favours the principal. In many money obligations, interest runs only from demand or default; here the law fixes it at the day of misuse, because the agent's wrong is using the principal's money as his own from that moment. He had the benefit of the funds from day one, so he pays for that benefit from day one. This both compensates the principal for being kept out of his money and removes any incentive for the agent to hold and use the funds quietly, gambling that interest would only start if and when the principal complained.
The second limb: sums owed after the agency ends
The article has a second part covering the wind-down. The agent also owes interest on those which he still owes after the extinguishment of the agency. So when the agency ends and the agent still holds money belonging to the principal — a balance not yet returned — interest runs on that outstanding amount too. This catches the agent who, once the agency is over, sits on sums he should hand over. He does not get an interest-free grace period after the relationship ends; while he retains the principal's money, it keeps costing him interest.
If your agent used your money
If your agent applied your money to his own use, you are owed not just the money but interest on it — and the clock started when he diverted it, not when you found out. So establish the date he began using the funds for himself and the amount, because those fix what interest is due. The same applies to any balance he has failed to return now that the agency is over; interest runs on that too. Keep the records that show what he held, when he used or should have returned it, and how much, because the interest is calculated from those dates.