Presumption of the conjugal nature of property

Dear PAO,

My grandparents were married in 1970. They are now separated-in-fact and my grandfather would like to sell the land, which he claimed to be his exclusive property. My grandmother opposes the disposition and warns the prospective buyer that the land is conjugal property, so her signature or consent is necessary. She presented a deed of sale executed during their marriage to prove that the property is conjugal. My grandfather, however, claims that the deed of sale is not sufficient, and my grandmother needs to prove that the land was acquired using conjugal funds. Is the deed of sale sufficient to prove that the land is a conjugal property of my grandparents?