Question

If I had not told him the truth , he would have signed the document. ( Begin : had)​

Answer 1

Had if I not told him the truth, he would have signed the document.

Answer 2

Had I not told him the truth , he would have signed the document .

DETAILS :

The meaning should NOT change.

The word to replace is "if" .

The last few words remain unchanged.

Hence the answer is given above and is correct I hope.