Begin as follows:
I received a contract from a university, but I do not like the salary. So I want to apologize in a good way and ask the principal to think again about the offer given; I would like a formal, respectful way of replying. Thank you very much.
In other words, write briefly and to the point, weigh every word and check whether it really means what you think it does in a dictionary, check your spelling, don't use contractions or colloquial slurring, punctuate carefully, and remember that saying "I want! I want!" for what you are demanding from others is for some strange reason considered impolite. And remember to say thank you for what you're offered.
PS. If you post your draft here, many people will be happy to offer suggestions as necessary.