Short answer. If you rightfully refuse goods delivered to you, Article 1587 of the Civil Code says you are not obliged to send them back. It is enough that you notify the seller of your refusal. But if you take it on yourself to keep and care for them, you become liable as a depositary.
What the law says
Unless otherwise agreed, where goods are delivered to the buyer, and he refuses to accept them, having the right so to do, he is not bound to return them to the seller, but it is sufficient if he notifies the seller that he refuses to accept them.
Civil Code, Article 1587 — Buyer's Refusal to Accept. Read the full provision →
Everything turns on having the right to refuse
The article opens with a condition that is easy to skim past: the buyer must be refusing having the right so to do. A refusal is rightful where the goods do not conform to the contract, where the wrong quantity or the wrong article was sent, where delivery came outside the agreed time or place, or where the seller tries to deliver in instalments that were never agreed. A buyer who simply changes their mind has no such right, and refusing then is a breach that exposes them to the seller’s claim for the price or for damages.
Notice is the whole of your duty
Where the refusal is rightful, the Code deliberately keeps the burden light. You do not have to arrange freight, pay to ship the goods back, or deliver them to the seller’s warehouse. Notifying the seller that you refuse to accept them discharges you.
Give that notice promptly and in writing, describing the defect or the discrepancy and telling the seller where the goods can be collected. Silence is dangerous: keeping goods without objecting, using them, or reselling them can amount to acceptance by conduct, and once you have accepted, your remedy shrinks to a claim for damages rather than rejection.
If you decide to look after them anyway
The second sentence is the trap. If you voluntarily constitute yourself a depositary — storing the goods, insuring them, moving them somewhere safe — you are liable as a depositary. That means a duty of care over property that was never yours, and responsibility if it is damaged or lost while under your control.
The phrase unless otherwise agreed also matters: a supply contract may impose its own return procedure, and a written agreement will usually override this default. Read the delivery and rejection clauses before acting, keep the goods untouched and documented, and make clear in your notice that you hold them at the seller’s risk and expense.