Is it safe to pay fully after signing the assignment deed?

I entered into a MoU with a seller who wants to sell his under-construction property, token amount already paid plus some additional amount paid (for his daughter education related). Seller is saying that deed of assignment will be signed, at which point I should share with him DD for paying his bank loan, balance amount to builder and finally his balance payment. My question is that is it safe to pay him off fully just after signing of assignment deed. I have never seen assignment deed before, what is the role of builder here? Is there going to be any subsequent sale deed with builder also or only the assignment deed is sufficient? Can you please advise how to mitigate any risk here?