Use "imposition" to refer to a tax or other unshakable burden, especially one that is excessive and uncalled for, as in, "The imposition for having 60 hogs on the farm rather than 50 was so steep that Farmer John had to sell his truck to pay it."
Use "imposition" to refer to a nonfinancial act of imposing, as in, "Aunt Mildred's imposition on our hospitality did not endear her to the family dog."
Use imposition to refer to a specific thing that is being imposed, as in, "The imposition of the death penalty on the serial thief was the source of much controversy."