Real Madrid's all-time highest scorer, who is due to play in Portugal's opening World Cup match against Spain on Friday, is the latest in a long line of top soccer players to be accused of tax evasion since a crackdown by Spanish courts.
The source confirmed Spanish newspaper El Mundo's report of the deal but said it had yet to be signed by the lawyers.
The 33-year-old is accused of evading 14.7 million euros in taxes. He has denied the allegations.
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Under the deal, Ronaldo would get a two-year jail sentence, but is unlikely to go to prison as Spanish law states a sentence of under two years for a first offence can be served on probation.
Barcelona's Lionel Messi was handed a 21-month prison sentence in 2017 on similar charges but under Spanish law was able to exchange the penalty for a fine.
Ronaldo clouded fans' delight in Real's 3-1 win over Liverpool in the European Champions League final in Kiev in May by hinting he was about to leave the club.
The five-time Ballon d'Or winner, was named the world's most popular athlete by ESPN on the basis of his social media following, endorsement deals and search score. He also headed the ESPN World Fame 100 list for a third year, pipping U.S. basketball player Lebron James and Messi.
(Additional reporting by Paul Day; Writing by Jesús Aguado; Editing by Julien Toyer and Editing by Robin Pomeroy)
