Could you please tell me if both of the following sentences are natural?

They offered him the post of Manager.

They offered him the post of a manager.



PS: Is it necessary that we use manageress for a woman?
I prefer the first version.

It is neither necessary nor desirable to say "manageress." (I am tempted to try to convince you that a woman manager is a "womanager," but I will resist the temptation.) A female manager is just called a manager.

Offhand, I can't think of any words that still use "-ess" except for "waitress" and "lioness". I don't think of myself as a "moderatress" on English Forums, or a "Jewess." Some people do still say "stewardess," but the airlines now call them all "flight attendants."
Thanks, Khoff.

What about "actress"? Do you find this word natural? Or do you prefer "actor" for women also?

Students: Are you brave enough to let our tutors analyse your pronunciation?

Princess Emotion: smile

And of course, where I work, I tell people I'm the brochure goddess!
I knew I was going to regret something in that post! Yes, "actress" and "princess" are certainly fine -- although I think some actresses prefer the word "actor." I'm sure other -ess words will come to mind as well.
Students: We have free audio pronunciation exercises.
That "Jewess" one made me think. Can you imagine a girl saying "His mother doesn't like me because I'm a Protestantess."?

Why, of all religions, would that one even have had an -ess?

Oh well, more a question for Topic of the Moment, I guess.