Dear Sir,
You were kind enough to share with me your doubts concerning the contract. You made it clear you believe it impossible for a man to turn into a nekker. You even drove the point home with a humorous comparison, saying, I believe, "same as a pig won't turn into a horse, not even if you strap a saddle on it and call it Black Beauty."
It is entirely possible that you are right. But if there exists even the shadow of a chance that my son has been imprisoned in the body of that monster, that some spell or curse has done this to him, then I will not rest until I try every possible way of helping him.
That is why I would ask you to refrain from further attempts at humor and keep your doubts to yourself. Instead, channel those energies towards doing the job for which you shall be paid. Catch the nekker, and collect your gold. The rest should not interest you.
Respectfully yours,
-Robert de Mere