The leg's force forward on the foot= 500n
the foot's force forward on the ball= 500n
the ball's force back on the foot= 500n
what is the net force on the ball?
what is the net force on the foot?

There's so much going on here, in a short period of time.

Before the kick, as the foot swings toward the ball . . .

-- The net force on the ball is zero.  That's why it just lays there and
does not accelerate in any direction.

-- The net force on the foot is 500N, originating in the leg, causing it to
accelerate toward the ball.

During the kick ... the 0.1 second or so that the foot is in contact with the ball ...

-- The net force on the ball is 500N.  That's what makes it accelerate from
just laying there to taking off on a high arc.

-- The net force on the foot is zero ... 500N from the leg, pointing forward,
and 500N as the reaction force from the ball, pointing backward. 

That's how the leg's speed remains constant ... creating a dent in the ball
until the ball accelerates to match the speed of the foot, and then drawing
out of the dent, as the ball accelerates to exceed the speed of the foot and
draw away from it.