Why the written form matters
Under article 733 of the Civil Code, a loan contract between citizens must be made in simple written form if the sum exceeds ten times the base calculation amount (BHM). If one party is a legal entity, the contract is written whatever the sum. The contract counts as made in written form if there is a receipt from the borrower, or another document that confirms the sum was handed over. If the written form is not kept, under article 109 the contract stays valid, but in a dispute it cannot be confirmed by witness evidence. Written and other evidence can be used.
The minimum content
State the following in the receipt: the full details and passport numbers of the lender and the borrower; the sum in figures and in words; the date the money was handed over; the repayment date; the signature. Always write the repayment date: under article 735, if no date is set, the borrower must repay the loan within thirty days from the day the lender demands it.
Interest and penalty
Under article 734 of the Civil Code, the lender has the right to interest in the amount and by the procedure the contract sets. So state the interest clearly in the contract. An agreement on a penalty must be made in written form (article 262). Under article 736, if the loan is not repaid on time, interest is paid in the amount set by article 327 from the day repayment was due until the day of actual repayment, whatever the contractual interest, unless a law or the contract sets another rule. If the loan is repaid in parts and one part is late, the lender may demand early repayment of the whole remaining sum with interest.
How to hand over the money
Under article 732, the loan contract counts as made from the moment the money is handed over. Under article 737, the borrower may challenge the contract by proving that they did not actually receive the money. So where possible pay by bank transfer, and write "under the loan contract" as the purpose of the payment. If you pay in cash, do it in front of witnesses and against a receipt.
On repayment
Under article 257 of the Civil Code, the lender must return the debt document when accepting repayment, and if that is impossible, must note this on the receipt. At the debtor's demand the lender must issue a receipt for the money received. So take the receipt back, or have the lender write "received in full" on it with the date and a signature.