Highly-rated Butler has been key in United's promotion this season, forming a formidable partnership with Andy Crosby.
He has won a host of admirers, and was linked with a £1million move to Premiership side Manchester City last week.
QPR boss Ian Holloway has been seen at recent games, and it was no surprise to see his side make an offer.
The Iron have yet to make an official annoucnment on the bid, but if he is to leave Glanford Park this summer then it is likely to be for a much larger sum.