The 32-year-old re-joined his boyhood club on Friday on a contract until the end of January and is now aiming to make brief impact in the Scottish Premiership before heading to Philadelphia to start the next chapter of his career.
He previously spent two weeks on trial with the MLS club in October after being released by St. Johnstone in the summer but failed to earn a contract at the time, despite leaving a positive impression on the coaching staff.
However, the veteran attacker has since held off signing a long-term deal elsewhere after being assured that the team remain interested in signing him for the 2016 campaign and is hoping to resume talks upon completion of his third spell at Fir Park.
"I went over, I had a trial, they were very happy with how it went and at the time they couldn't sign me even if they wanted to," McFadden told the BBC. "It is a case of just waiting. They've got a few things going on in terms of getting rid of some players and then hopefully we'll get round the table to agree a deal for January.
"I held off signing anywhere until I knew what was happening as that's what I wanted to do."
McFadden, who earned 48 Scotland caps between 2002 and 2010, scored just twice in 20 appearances for St. Johnstone last season, but with his Premier League and international pedigree, he could be an intriguing addition for the Union.
The club are looking to rebuild their roster this winter after a disappointing ninth-place finish in the Eastern Conference and will need to bring in some attacking reinforcements over the coming weeks and months after trading both playmaker Cristian Maidana and winger Andrew Wenger to the Houston Dynamo last week.
Scottish imports like Kris Boyd, Shaun Maloney, Kenny Miller and Barry Robson have not fared too well in MLS in the past, but if Philadelphia can work out a deal for the right price, then McFadden just might end up proving worth the investment.