The British Airways aeroplane which lands in "Chicago" is a Boeing 737, a type not normally used on long haul, transatlantic flights (certainly not by BA).
As the film starts it is clear that the airplane coming into land is landing at London Gatwick in England, you can see the M23 motorway in the distance where the traffic is clearly driving on the left hand side. we then see Sean Bean walking through an American airport then into an American car and driving down an American road.
While the three were calculating how much was spent, and therefore still owed by Sam and Leslie, the $4.00 Leslie gave to the mortgage officer before she even knew about the "found" money was never credited back to them. They included it into what they had spent. Meaning they "found" $625,727.00, not the $625,731.00 that was used as the basis for all the calculations.