The 'send a payment to your friend because it will be quite safe ploy' is only used by scammers to steal money.
It doesn't have any other function. It doesn't do anything else other than to enable him to steal your cash. If he only wanted to see that you have the cash, you could send him a copy of your bank statement
but he wants your cash in his pocket.
He will insist on the MTCN number and tell you any lie to try to persuade you that he can't do anything with it - but that's just a plain old-fashioned lie.
If a UK property owner doesn't have a UK bank account in their own name, then you're dealing with a scammer, and sending money by Western Union to someone you don't know is a certain way to end up never seeing it again, or getting anything for your cash.