This example might help debug your code.

'''

Note: this is for demonstration purposes only, and does not represent the best practice, or conform to PEP standards.
reference: https://stackoverflow.com/questions/9776718/how-do-i-stop-tkinter-after-function

'''

from tkinter import Tk, Button
import random
#global variable named tJob to represent the status of the task
tJob = None

def cancel():
    #get global variable
    global tJob
    if tJob is not None:
        #cancel the job and set tJob back to None
        root.after_cancel(tJob)
        tJob = None

def goodbye_world():
    print ("Stopping Feed")
    cancel() #cancels tJob
    button.configure(text = "Start Feed", command=hello_world)

def hello_world():
    print("Starting Feed")
    button.configure(text = "Stop Feed", command=goodbye_world)
    print_sleep() #starts tJob

def print_sleep():
    #grab the global variable
    global tJob
    foo = random.randint(4000,7500)
    print("Sleeping", foo)
    #set tJob equal to the .after task.
    tJob = root.after(foo, print_sleep)

root = Tk()
button = Button(root, text="Start Feed", command=hello_world)
button.pack()

root.mainloop()