It's a presumption that the transfer is taxable income and a requirement to demonstrate otherwise. That's a very heavy tax enforcement process, but in theory you can get the 20% back if it's not income. If it is income, you should have paid the 20% anyway.
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Addendum: this appears to be somewhat true:
via http://www.zerohedge.com/news/2014-02-16/money-launderer-unt... original italian news http://www.ilsole24ore.com/art/notizie/2014-02-13/ritenuta-a...
It's a presumption that the transfer is taxable income and a requirement to demonstrate otherwise. That's a very heavy tax enforcement process, but in theory you can get the 20% back if it's not income. If it is income, you should have paid the 20% anyway.