You think this would not work? Is there a better way to show to someone you really have the amount ot BTC you say you do in your wallet?
you can go to your wallet choose the address that contains funds you want to prove they belong to you then sign a message from that address with its private key.
for instance:
bg17aw owns this address and the date is 2018-07-05
this way you don't have to spend the coins to prove anything, it is easy and fast and also nobody can fake it. and more importantly anybody can verify it easily.
here is the way to do it:
https://bitcointalk.org/index.php?topic=990345.0