The La Liga side were interested in securing the France international on a free transfer, with his contract in Paris set to expire at the end of June.
The 23-year-old is said to want €15 million per season in wages, with the total costs of a deal amounting to around €20 million per year.
Those requests have seen Betis drop out of the race and make United favourites to secure the services of the player, according to Football Espana.
Rabiot hasn’t featured for PSG in almost half a year after being frozen out as he entered the final months of his contract.
Other clubs said to be interested include Tottenham Hotspur, Arsenal, Liverpool and Real Madrid.
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