My take on it is. Assuming its residential property, commercial property is different as you can opt to tax.
If you cancel your waiver before sale then you have a cancellation adjustment; VAT reclaimed - VAT repaid. €50,000 - €20,000 = €30,000.
If you sell the property prior to cancelling the waiver then you have a Capital Goods Scheme adjustment VAT reclaimed x (number of years plus 1 of life left /20). €50,000 x 12/20 = €30,000 followed by a cancellation adjustment.
€50,000 - €20,000 - €30,000 = 0
So in your case I don't see a difference in cancelling the waiver prior to sale or at the point of sale as both are €30,000.