I recieved a contract from a university but I dont like the salary So I wanna to appologize in a good way and I ask the principle to think again about the offer given, I wanna a formal respective way of replying
This can be pretty tricky because you want to sound greatful but at the same time inform them that you were expecting a different salary.
I might even suggest doing this in person. If you go and express your interest but then your reluctance to accept due to the salary or finacial side of things that will show you are very interested (enough to go in person) and that you are taking this seriously. It also puts them a bit on the spot and you might be able to get a bit more money from them...although probably the person you go to see will have to get things approved by others.
I don't want to make a recommendation for writing the letter because its not my area of knowledge but I am sure someone will help you out.
Best of luck,