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.