I got a letter from a man postmarked from Spain. He wants me to act like the beneficiary to his dead client. So he will need me to write a letter of authorization and sign it. Then he can present me as the relative and get the money. Well he is not asking for anything else but this. He said he will provide all the documents to me once the signature is to him and he can get the ball rolling. The money will go to me and then he asks for half from me when I get it. Is this a scam and how could he use my signature to hurt me?

Very common scam, and makes you part of the crime

First you are going to do a letter and lie that you are a beneficiary, ( not that it really matters since there is no dead person and no real money)

They send a check, normally stolen or fraud from some real account. You deposit it in, your bank, and since it is a real check often just stolen or fraud ( they make fake check off someone's real account in many cases)

And then they want you to send by Western Union their share.

Then about 2 or 3 weeks later, the check comes back as bad, so you owe the bank the entire amount back,
You already sent 1/2 and can not get it back,
And the bank can press criminal charges ( esp if you don't pay all the money back)

The way it works, they find people so greedy they will not have a issue lying that they are a person who should get this money.

I said up front I will never give them money or ss# or bank numbers. He is sympathetic to my charity causes. He wants my signature, can he get me in trouble and use it for something?

This is old scam...