I suggest you sign an agreement with your relatives first, and make it clear that you will formally go through the formalities of buying and selling real estate in five years. Both parties agree that the house payment cannot be changed, and the house payment has been paid in full, and the amount of the house payment cannot be changed when going through the purchase and sale formalities after 5 years. Then you can contact a notary office, where you and the seller will be present together to notarize the property sale agreement five years later. I recommend you to contact Nanjing Zhongshan Notary Office. They have a good service attitude and can help you advise on how to notarize.
Address: Nanjing Zhongshan Notary Office, 4th floor, Yingjia Building, No.369 Zhongshan South Road, Nanjing, Tel: 58074603, 58070603.