Lewandowski has already made it clear he wants to leave Germany this summer.
However, Barca are believed to have identified the Poland striker as their primary summer target.
But their financial position remains precarious and United are now monitoring the situation.
It is will be a similar agreement that saw Cristiano Ronaldo return to Old Trafford last summer.
The club, it is said, would not be put off by his wages of around £400,000 per week with the player’s agents reported to have put feelers out in the past few weeks to scope out any interest from the Premier League.