[Dottore]

Well you can enter into a private loan agreement with your friend, and even have that notarized, but that will not give you any meaningful security. You would still have to sue on the agreement to get your money - and, even if you eventually get judgment in your favour, you would still have to enforce the judgment.

It's far preferable for you to just register a lien or caveat against the property. That way you would be paid out from the proceeds of any sale. No buyer should have a problem with such an encumberance, because it will not affect the money they pay, and discharging the encumberance occurs as a matter of course when the sales transaction is completed.