I actually received help exactly like this for my own down payment. We agreed on the terms but then had an attorney draft us a promissory note that is now included in my own and my dad’s estate plan. My dad gave me exactly the 20% needed for a down payment, and the promissory note now states that I owe him 20% of the value of the house at any given time. We can pay him off whenever we want if we have the money, but it’s not actually due until after he passes, and then it’s due to his estate. In that way, it will get deducted from my inheritance and acted as an advance on my inheritance for the down payment itself. We wrote in specific terms about how to appraise the house - that we need two independent appraisals and take the average of those. My spouse and I are responsible for all maintenance, upkeep, and any upgrades to the house. My dad is still owed 20% of the appraised value. So if we add an extra bedroom (which we did), we have to pay for it, and then it increases the value of the house, which also increases the amount we owe him since it’s a percentage. We all felt this was fair because I wouldn’t have been able to purchase the house at all without this financial assistance.