Short answer. You can demand cash from the agent himself. A commission agent may not sell your goods on credit without your express or implied consent, and if he does, you may require him to pay you as though the sale had been for cash. Any interest the credit terms earn, however, is his.
What the law says
The commission agent cannot, without the express or implied consent of the principal, sell on credit. Should he do so, the principal may demand from him payment in cash, but the commission agent shall be entitled to any interest or benefit, which may result from such sale.
Civil Code, Article 1905 — Sale on Credit Without Consent. Read the full provision →
What the article gives you
Article 1905 is short and unusually practical: The commission agent cannot, without the express or implied consent of the principal, sell on credit. Should he do so, the principal may demand from him payment in cash, but the commission agent shall be entitled to any interest or benefit, which may result from such sale. The sale itself is not undone. What changes is where you look for your money. Instead of waiting on a buyer you never chose and could not assess, you may require the agent who granted the credit to pay you now, in full, in cash.
Consent may be express or implied
You need not have signed anything for the agent to be covered. Consent can be implied — drawn from how the two of you have dealt before, from terms you knew about and never objected to, or from a settled practice in the trade that you were content to follow. Equally, a principal who learns of the credit sale and then accepts it, collects on it, or treats it as his own will find it hard to demand cash afterwards. The safe course for both sides is to state in writing whether credit is allowed at all, and if so up to what amount and for how long.
Why the agent keeps the interest
The closing clause looks strange until you see the trade-off. If you take cash from the agent immediately, he is the one left carrying the buyer's debt and the risk that it is never collected. The article gives him any interest or benefit, which may result from such sale because he has, in substance, bought that receivable from you. You get the full price now; he gets whatever the credit terms yield, and the sleepless nights if the buyer defaults. It is an allocation of risk between two people, not a reward for acting without permission.
What the article does not cover
It does not deal with an agent who sold below the price you set, sold to himself, or disposed of goods he was never authorised to sell — those raise separate questions. It does not cancel the buyer's purchase, so a buyer who dealt honestly is not caught by this rule. And it fixes no deadline for the agent to pay. In practice, put your demand in writing, state plainly that you require payment in cash, and keep the record of the goods delivered and the terms you gave him. Claims for sums of money are subject to time limits, so do not leave it for years.