The mistake almost everyone makes when lending money to family

How it started: 12,000 euros via Bizum

My brother had a chance to buy a cheap car from a colleague. He needed the money that same week, before it got sold to someone else. I had 12,000 euros sitting in a savings account that had been idle for months. I told him: no problem, I will lend it to you, pay me back whenever you can. I sent the money over Bizum (yes, twelve thousand euros over Bizum, in batches so I would not hit the daily limit) and went on with my life.

Three weeks later the first letter from the Spanish tax agency arrived asking about the transfers. Three years later the matter was resolved, but with pain. These are the five mistakes I made, in the order that matters most.

Mistake 1: Not filing Modelo 600

Modelo 600 is the Spanish stamp duty form. Private loans are subject to this duty but exempt when no interest is charged (art. 45.I.B.15 of the TRLITPAJD). In other words: if there is no interest, you do not pay a cent, but you still have to file the form within 30 days of the loan.

What if you do not file it? When Hacienda detects the transfer (which they will: banks report any amount above 3,000 euros), they will presume it is a gift, not a loan. Sibling-to-sibling gift tax in many Spanish regions (Catalonia, Valencia, Asturias, Aragon at the top brackets) sits at 7-32%. On 12,000 euros that could be 800 to 3,800 euros in tax. Plus a 50% penalty if Hacienda determines concealment, which it will: no form was filed.

Lesson: file Modelo 600 within 30 days of the loan, even if the resulting tax is zero. It is free (you can file it on the regional tax website or pay any accountant 30-50 euros) and blocks the gift presumption.

Mistake 2: Not dating the document (because there was none)

The first question Hacienda asked: when was the loan signed? I had no paper. I told my accountant we had agreed verbally and then made the Bizum transfer. Without a certain date, Hacienda can reclassify the operation as a gift at any moment.

The certain date of a private document is established by: (a) presentation before a notary, (b) presentation before a public registry (including Modelo 600), (c) death of a signatory, or (d) handover to a public servant on duty (art. 1227 of the Civil Code). Modelo 600 establishes a certain date as a side effect, one more reason to file it.

Lesson: dated written contract plus Modelo 600 filing are two birds with one stone. Without one, the other loses strength.

Mistake 3: Not specifying terms or instalments

Whenever you can is the most expensive phrase in the world between relatives. Six months passed. My brother never mentioned the money, neither did I. Hacienda asked how much had been repaid. Zero. Bad signal: if nothing comes back in six months, it looks like a disguised gift.

The practical rule for the tax agency: for a family loan to look like a real loan, money must move back. Even 100 euros a month symbolically in the first year while the situation stabilises. There has to be a regular bank trail of repayments.

Lesson: in the contract set specific repayment dates (monthly, quarterly, yearly), even small amounts. Set up a recurring standing order or direct debit from day 1. The movement is proof.

Mistake 4: No witnesses or notary (at least a crossed email)

You do not need a notarial deed for a 12,000 euro family loan (it would be expensive and over the top). But it helps to have extra proof that both parties agreed to the same thing. In my case: no email, no clear WhatsApp message (just a vague ok, sending), no witnesses around a table.

The minimum viable form: after signing the contract, send each other an email confirming the loan, amount and instalments. This creates metadata (date, time, IP, server) which is huge evidence in court. If the amount is high (over 50,000 euros) a notary becomes worth it for the certain date and permanent archive.

Lesson: contract plus mutual confirmation email plus Modelo 600 equals three layers of proof. Any inspection facing those three falls flat.

Mistake 5: Not each party keeping a signed copy

This is textbook and yet almost nobody does it. If the contract exists but only one party has it, the other can deny having signed it or claim the text is not the original. Print two copies, sign both, each party keeps one. If electronic, both sign with their qualified digital certificate or a recognised signature system (a PDF with a scanned signature is not enough).

Bonus: with platforms like Runican today you get a PDF with SHA-256 hash and a verification QR code. That is immutable proof that this exact text was signed, no room for argument.

Lesson: two signed copies, one per person. If digital, with cryptographic signature.

How the story ended (spoiler: with luck)

Three months after the first letter from Hacienda, my accountant filed a loan contract signed with retroactive effect (legally debatable but in practice accepted if no penalty is final yet), the late Modelo 600 with a 10% surcharge, and a written explanation. My brother started repaying 200 euros a month via transfer with concept Loan repayment. The inspection closed without additional penalties but with the surcharge.

Total cost: 0 euros in tax (no interest), 78 euros surcharge for late filing, 250 euros to the accountant, several weeks of stress. All avoidable with a piece of paper and a 30-minute procedure at the right time.

And between my brother and me, did anything happen? No. But because we were lucky: he accepted the new schedule, I did not throw it in his face, we both talked about it. What I learned is that this kind of luck is built with a signed paper from day 0.

Final checklist if you are about to lend money to a family member

1. Written contract with amount, instalments, term, no interest (art. 1755 of the Civil Code).
2. Two signed copies, one per party (or recognised electronic signature).
3. Modelo 600 filed within 30 days even when the tax due is 0 euros.
4. Bank transfer (not Bizum, not cash). Clear concept: Loan to X.
5. Repayment via recurring transfer. Concept: Loan X repayment.
6. Mutual confirmation email after signing.

Thirty minutes, zero euros. What you save: potentially thousands in tax and penalties, and some awkward conversations with family.

What my case taught me about family dynamics

Beyond the tax angle, the hardest part was the conversation with my brother. When I called him after the first letter from Hacienda, his first reaction was defensive: you always make everything complicated. It took us four days to talk calmly. What I learned:

• The paper is not distrust, it is prevention.Reframing the conversation as this protects both of us from the system instead of this forces you to pay me changes the dynamic.
• Future transparency is agreed today. In the annex to the contract (signed three months later) we added a clause that literally read if either of us hits serious financial trouble, we will say so before missing a payment and renegotiate the calendar. That sentence saved us three uncomfortable conversations later on.
• The monthly movement is psychology, not just tax.Watching the 200 euros come in each month is not something I need (I park it in a savings account), but it creates consistency and discipline for both parties. It is like a small monthly subscription to your family relationship.

If you only read one section of this post, let it be this one: the family contract is not a bureaucratic act or a sign of distrust. It is an adult way of handling money between people who love each other.

Related reading

• The full guide on protecting yourself when lending to family
• Family loan without taxes (Modelo 600)
• Contracts you should have before 30