Boss Alex Ferguson has offered Tottenham a £25million fee, rising to £30m based on appearances and success.
Modric, 26, will be handed a four-year deal worth £26m.
His wages of £125,000 a week will put him among the big earners at Old Trafford.
Modric has kept up to date with talks while at Euro 2012 with Croatia and will seal the move on his return.
Spurs held off interest from Chelsea to hold on to the midfielder last year.
Can't see Tottenham accepting the bid.
Chelsea offered way more and got turned down..
Even 30 mil is too much for him. I don't see what the fuss is about. He's probably worth 20 mil.
I just hope the allure of playing for utd a club that has won numerous european trophies will persuade him to come.
Can't see Spurs accepting £25 million when they rejected £40 million from Chelsea only a year ago. It will be difficult for them to keep hold of Modric though without Champions League football for a second year running.