You should definitely not sign the offer letter if you were expecting a different offer. The offer letter is what they are offering, no matter what was said to you previously. However:
Don't assume malevolence where a mistake is a valid explanation.
Return the contract with a simple statement that the salary in the contract is wrong. Remind them of the salary you were offered. Don't accuse anyone of anything or make any statement about how you would respond to an offer of that salary.
The advantage of this is that if the change is a genuine mistake, you maintain good relationships with the company. If it isn't then you will get at least some explanation of why there was a change, and you still come off as a reasonable person, and you haven't lost anything. (Doing this is also an indication to them that you don't like the new salary and won't take crap, without actually having to say so.)
If they come back and say that the conditions have changed, or that the original offer was a mistake, or the job is different, or something similar, then you can make your decision about whether you want the job at the new salary and whether you trust the company. I personally wouldn't trust them. A trustworthy company, when doing something like this, starts by saying "I'm sorry but the responsibilities of this job has changed" or "the salary we offered you was a mistake", or something like that. They don't just make changes and hope you don't notice.