Land sale agreement and seller's warranties
The sale agreement for land includes important provisions protecting you as the (indirect) purchaser of the land and should prevent unexpected issues after the transfer of ownership. I can imagine that they do not want to use the sale contract because they do not want to guarantee anything or give any warranties concerning the land or transfer of ownership.
It could also be that they do not want to use the contract because it is a hand shake deal between family members. Personally I do not like these you can trust us deals between family members where foreigners are involved who must pay for the deal. But if it is a small investment and it makes your wife happy you can skip the contract part? If it is a larger investment I would stay away from it (with or without a contract). The seller may conceal some matters concerning the land, land title or transfer of ownership which are mentioned as the seller's warranties in the downloaded land sale contract and which he obviously does not want to guarantee as it could lead to liabilities for him after the transfer of ownership of the land.
When you buy land on the name of your wife she will be the sole owner, it cannot become a marital asset as foreigner cannot own land in Thailand
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