“There are two places in the tractate ( 35b; 54b) that indicate that the purchase of real estate from a gentile is not completed with payment alone, without the additional writing of a sales document as proof of the sale.

Source: https://www.jewishpress.com/judaism/halacha-hashkafa/the-halacha-of-purchasing-mearas-hamachpela/2023/11/10/