The 24-year-old goalkeeper has been in excellent form for loan club Aston Villa this season. His performances have been pivotal for Villa's surge up the Championship table.
Johnstone has stated his preference to remain at Villa Park. And The Sun have revealed that the Midlands club will have to fork out £6.5m for the United academy product.
Last week, the Red Devils took up a 12-month option on Johnstone's contract to ensure they could make money off his sale.
The Englishman is currently behind David De Gea in the United pecking order. But if the Spaniard decides he wants a move to Real Madrid, Jose Mourinho will be expected to be a world-class replacement rather than Johnstone.
Johnstone said last week: "I want to keep playing football. I'm enjoying the number of games I'm playing.
"The Premier League is where you want to be week in, week out. I'd love it for this club to be a part of that next season."