Celtic claim a fee was agreed with Manchester City for John Guidetti prior to them taking the striker on loan.
Xabi Alonso claims no Premier League offers were put to him before leaving Real Madrid for Bayern Munich.
Schlupp has been training with the Barclays Premier League leaders since the middle of January and this second extension to the arrangement will see the 20-year-old striker finish the month with Sir Alex Ferguson's side.
"He's stayed there, we've agreed to extend that," Leicester boss Nigel Pearson told Press Association Sport.
"I have spoken to him a few times and Jeff is enjoying himself there, things
are going well. I think it's great experience for the lad."
The move is the latest in a burgeoning relationship between the npower
Championship club and United, which has seen Ritchie De Laet and Danny Drinkwater sign for City after leaving Old Trafford and Michael Keane and Jesse Lingard arrive on loan from the 12-time Premier League champions.
Schlupp, who came through the youth ranks at Leicester, has made five starts this season but the majority of them have been at left-back as cover for Paul Konchesky.
The Germany-born Ghana international also made an impact at Brentford during a loan spell in 2011, scoring six times in 10 appearances.