Buying the car that is still under finance - What are the steps?

I am really sorry that I have to write in English, but I don't know any other place to ask.

I want to buy the car from a private seller. We agreed that he will sell it to me for 200,000b. The car is still under finance, and he wants me to pay 100,000b to close his deal with the financing company first. Then he will transfer the car to me and I will pay the rest of the money 100,000 more.

He said that everything will be according to law, that we gonna sign a contract stating that I paid for the closing of the balance.

I don't know anything about this, and I don't want to get a scammed.
I think that he owns the car, but that the company keeps the blue book with them until he pays off the financing money.
Car has white plates and its 9 years old.

Can anyone please explain me what is the proper procedure to do this, and which documents we have to sign in order that everything goes well.?

My Thai friend will be with me, and he said that this kind of transfer happens a lot in Thailand and that is normal. I trust my friend but I am still concerned for some reason. (maybe cause I don't know the proccess)

Any help appreciated.
โปรดศึกษาและยอมรับนโยบายข้อมูลส่วนบุคคลก่อนเริ่มใช้งาน อ่านเพิ่มเติมได้ที่นี่