Short answer. Only what you actually benefited from. Article 1399 of the Civil Code protects an incapacitated party: when the defect is incapacity, that person is not obliged to make any restitution except insofar as he has been benefited by the thing or price received. You do not owe back more than the benefit you actually got.
What the law says
the incapacitated person is not obliged to make any restitution except insofar as he has been benefited by the thing or price received by him
Civil Code, Article 1399 — Restitution by an Incapacitated Party. Read the full provision →
The restitution rule for incapacitated parties
When a contract is annulled, the general rule is mutual restitution — each party returns what they received. Article 1399 carves out a protective exception for the incapacitated party. The incapacitated person is not obliged to make any restitution except insofar as he has been benefited by the thing or price received by him. If you received money and spent it on necessities, your obligation is limited to what you actually got from it. If you received goods that you consumed, your obligation is measured by the benefit you derived, not by the goods' market value. The other party — who dealt with someone who lacked capacity — bears the risk of the difference.
Why the law draws this distinction
The purpose of Article 1399 is to protect people who lacked the mental or legal capacity to protect themselves when the contract was made. The other party dealing with an incapacitated person is presumed to know, or should have known, of the incapacity. It would be unjust to hold the incapacitated party to full restitution when the very contract that created the obligation was void or voidable because of their incapacity. The protection also prevents creditors of the incapacitated party from indirectly recovering more than the benefit conferred, through restitution claims dressed up as annulment.
Measuring 'benefit actually received'
The key factual question is how much you actually benefited. If you received money and used it to buy food, medicine, or pay for shelter — things that directly improved your situation — you benefited by that amount. If the money was squandered or lost without any advantage to you, your obligation may be zero or very small. If the property received was used productively or is still in your possession, the benefit can be measured more directly. Evidence of how the thing or price was actually used becomes essential in determining the extent of the obligation.
What this means for the annulment proceeding
In an annulment proceeding, the other party may claim full restitution of what they gave you. Article 1399 gives you the right to contest that. You can raise the incapacity and argue that your obligation is limited to what you demonstrably benefited from. The burden of proving what you received generally falls on the other side; the burden of showing what benefit, if any, you derived may require you to provide evidence about what happened to the thing or price. Legal representation in the annulment proceeding matters here, because the extent of restitution can be a contested issue with significant financial consequences.