Scam, western union is used mostly for scams.
Avoid at all costs & report them - if on the 1% chance you are wrong then don't feel bad - but personally, I'd consider that at the same level as the following.
HELLO MANSIR
I AM THE SON OF THE GTREAT PRICE ON NIGERIA, I NEED £50 TO PROVE YOU HAVE BANK ACCOUNT SO I CAN SEND YOU $150,000 POUNDS TO KEEP FOR MY UNCLE THE KING OF NIGERA.
IF YOU HOLD MONEY I WILL SEND YOU £250,00 FOR THE TROBLE.
PLEASE SEND BANK DETAILS TO THE FOLLOWING ADDRESS ALONG WITH THE $50 PAYMENT.
THANKS,
LORD KATIMO OTUMA III - CROWRN PRICE OF NIGERIA