Fergie has lost a succession of bets with his brilliant young star, including last term's 'Scotsman's bet' when he kept raising the barrier for Ronaldo, who eventually finished on 23.
However, Ferguson has no desire to lose any more cash and will not be asking the Portugal winger for the opportunity to win his money back.
"I will not be attempting to bet anything with Cristiano this season unless it is 50 goals," he laughed. "Somehow I don't think he would accept that."
A more productive gamble might be on Ronaldo getting a much-prized United hat-trick.
While he has now scored twice on 10 occasions, the former Sporting Lisbon player is yet to hit a three-timer and could not manage it against Blackburn yesterday despite the light-hearted promptings of Rio Ferdinand.
"In the second half I told Cristiano, he either had to score a hat-trick or get an assist to make it a really good game," Ferdinand said.
"He has been fantastic in the last few games and is determined to become a top player for many years.
"He works hard in training and you see the results out on the pitch. Long may that continue."