Short answer. Generally no. Article 1918 of the Civil Code provides that a principal is not liable for expenses incurred by an agent who acted in contravention of the principal's instructions. There is one exception: if the principal chooses to take the benefit of what the agent did despite the violation.
What the law says
The principal is not liable for the expenses incurred by the agent in the following cases: (1) If the agent acted in contravention of the principal's instructions, unless the latter should wish to avail himself of the benefits derived from the contract; (2) When the expenses were due to the fault of the agent; (3) When the agent incurred them with knowledge that an unfavorable result would ensue, if the principal was not aware thereof; (4) When it was stipulated that the expenses would be borne by the agent, or that the latter would be allowed only a certain sum.
Civil Code, Article 1918 — When the Principal Is Not Liable for Expenses. Read the full provision →
When the agent violates your instructions
Article 1918 of the Civil Code lists four situations where a principal is not required to reimburse expenses incurred by the agent. The first applies directly to your situation: If the agent acted in contravention of the principal's instructions, unless the latter should wish to avail himself of the benefits derived from the contract. If your agent departed from the instructions you gave and incurred expenses in doing so, you are not obliged to pay for those expenses. Your instructions define the scope of the agent's authority; actions taken in violation of those instructions fall outside what you authorized.
The exception: if you take the benefit, you bear the cost
The exception built into the first ground is important. If you decide to avail yourself of the benefits derived from what the agent did — that is, if you take advantage of the results of the agent's unauthorized action — you cannot also refuse to pay the costs. The law does not allow you to accept the gains of what the agent did while denying the costs. If the agent's unauthorized act produced a useful result and you choose to benefit from it, that choice makes you liable for the expenses that were incurred to produce that result. The decision is yours: reject the unauthorized act and avoid the cost, or accept the benefit and bear the expense.
The other three grounds for non-liability
Article 1918 lists three additional grounds where the principal is similarly not liable. Second, when the expenses were caused by the fault of the agent — if the agent's own carelessness or wrongdoing caused the cost, the principal should not bear it. Third, when the agent knew that an unfavorable result would follow but the principal was not aware — if the agent went ahead with a transaction knowing it would go badly, and the principal did not know, the agent cannot shift that self-imposed loss to the principal. Fourth, when the parties agreed that the agent would bear expenses, either entirely or up to a set amount — in that case, the contract already allocates the cost to the agent.
What this means for your situation
If your agent ignored specific instructions you gave and incurred expenses in the process, your starting position under Article 1918 is that you do not owe reimbursement. To maintain that position, you should be clear that you are not accepting or benefiting from what the agent did — if you take advantage of the outcome, the exception applies and you may become liable for the costs. Document your instructions clearly and, if the agent acted against them, make plain that you are not ratifying the unauthorized conduct. The agent's right to reimbursement depends on having acted within the scope of your authority; stepping outside that scope generally leaves the agent bearing their own costs.