For loans between relatives without fuss
It is natural to turn to family or friends in times of difficulty. However, precautions are necessary to avoid litigation … and the wrath of the tax authorities.
https://www.taxadvance.com/
Prioritize writing
The drafting of writing is a guarantee for the one who lends the money.
Moreover, when the loan exceeds €1,500, proof that it is a loan can in principle only be done in writing, even if the sums were paid by check or transfer.
You can write a loan agreement signed by both parties or an acknowledgment of debt signed by the borrower's hand alone.
https://www.nerdwallet.com/article/loans/personal-loans/tax-refund-loans-give-cash-now-to-early-filers
Whatever the writing is chosen, it must specify that it is a repayable loan, and indicate its amount, its duration, its terms of repayment (in one or more installments), the date of repayment, the rate of interest if any… If the lender collects interest, he must declare it as taxable income.
The acknowledgment of debts must always include the amount in figures and in letters written by the hand of the borrower.
https://smartasset.com/taxes/where-can-i-get-a-tax-refund-loan
When the loan is for a large amount (to finance the purchase of a home, create a business, etc.), it is better to ask a notary to draft the loan deed.
Registration (optional) of the deed of loan
If the loan is large, it is a good idea to register the deed of loan in order to establish without dispute the date on which it was drawn up – making the “date certain†in the jargon of lawyers – with the office of the registration (a tax service) subject to payment of a tax, known as a fixed fee of 125 euros.
https://turbotax.intuit.com/refund-advance/
The registration makes it possible to prove in the event of a tax audit that it is a real loan. Because when the origin of a sum cannot be proven conclusively, the tax authorities can tax it as an undeclared donation or as income of undetermined origin.
The (mandatory) declaration of the loan to the tax authorities
https://www.jacksonhewitt.com/refund-advance/
When the loan exceeds €5,000 (since September 27, 2020, compared to €760 before this date), the borrower is obliged to declare it to the tax authorities using form no. 2062 to be provided at the same time as his declaration of income to its tax department. But the taxes specify that "  when several loan contracts for a unit amount of less than  € 5,000 are concluded during a year in the name of the same debtor or the same creditor and their total exceeds 5 000 €, all contracts thus concluded must be declared “.
Non-declaration or a declaration containing omissions or inaccuracies is punishable by a fine of €150 ( article 1729 B of the General Tax Code ).
https://www.hrblock.com/offers/refund-advance/
If the loan is accompanied by interest, the borrower (the debtor) must declare the interest he has paid and the lender (the creditor) must declare it in his annual declaration of income, under the heading income from movable capital. With the exception of certain loans granted in 2006/2007, interest is taxable (flat-rate deduction or tax scale).
It is strongly advised to keep proof of reimbursements.
Prioritize family peace
Writing a contract or an acknowledgment of debt also helps avoid family quarrels, especially if the loan was granted to only one child, and the others received nothing. Know here that it is always possible to subsequently make a shared donation in favor of all the children by transforming this loan into a donation, in order to restore equality between heirs.