Done has twice had his fingers burnt by making such a move before, including when Sir Alex Ferguson's side were overhauled by neighbours City in the closing stages last term.
That decision cost the Salford-born bookmaker over £1million and led some United fans to accuse him of jinxing the Old Trafford outfit, having also paid the price for a similar move when Arsenal clawed back a huge deficit in 1998.
However, with United again a dozen points ahead of their nearest rivals following a 2-0 defeat of Everton, Done has seen enough to declare them champions for a record 20th time.
He said: "After it all went wrong last year, my missus made me promise to never do it again, but with United 12 points clear with just 12 games left, the title race is over - so my punters can collect their dough on United."
Paddy Power had already stolen a march on their competitors by paying out on United to finish top of the table, even before they had claimed victory over the Toffees.
A spokesman for the Irish firm declared: "The fat lady might not be singing yet but she's definitely getting warmed up.
"We can't see United letting their lead slip and, with Real Madrid to come in midweek, we thought we'd take some of the pressure off Fergie and give punters an early payday."