Making an NPC interrupt his path to follow player?

Just store your moveto values (the part and the position) in 2 variables just before you start following a player and put it back when you stop following (player moves out of range)?

Then you can use the player as an object to follow using Humanoid:MoveTo which makes your life a lot better to program that part.

If the reason you want to keep the original target in the humanoid, is because other scripts modify this target while you ignore it, then you will have to make an ObjectValue (say as a child of humanoid) or something that stores the target that these scripts can interact with.

Then the script that checks which target to follow connects a function to the Changed event of that objectvalue, and if not following a player, sets the new target using Humanoid:moveTo()

That way, it's easy to ignore the original target while always storing it in a safe place.

Example implementation (untested, just for example):

local humanoid = --find player's humanoid that you want to move
local objectValue = Instance.new("ObjectValue", humanoid)
objectValue.Value = --find original target to walk to
local following = false

objectValue.Changed:Connect(function(newVal)
  if not following then humanoid:MoveTo(Vector3.new(), newVal) end
end)

while true do
  local nearestPlayer = playerInRange()
  if nearestPlayer and nearestPlayer:FindFirstChild("Character") and 
  nearestPlayer.Character:FindFirstChild("HumanoidRootPart") then
    if not following then
      following = true
    end
    if not (humanoid.WalkToPart == nearestPlayer.Character.HumanoidRootPart) then
      humanoid:MoveTo(Vector3.new(), nearestPlayer.Character.HumanoidRootPart)
    end
  else
    if following then
      following = false
      humanoid:MoveTo(Vector3.new(), objectValue.Value)
    end
  end
  wait(0.5)
end

(of course add a function playerInRange() that returns the nearest player or nil if none in range)