@Andres: I can see much simpler way:

defmodule Example do
  def sample(integer) when is_integer(integer) do
    integer |> Integer.digits() |> Enum.reverse() |> Integer.undigits()
  end
end

Is that what you wanted?

Edit: Code changed after @Ted comment

Originally I noticed this on other data too, but I found that it is not a problem in this specific case, because as far as I know Integer.digits/1 will never return list like [1, -2, 3].

There is nothing surprising with 83 as result, because we have: (3 * 10^0) + (-2 * 10^1) + (1 * 10^2) i.e. (3 * 1) + (-2 * 10) + (1 * 100) i.e. 3 - 20 + 100 which gives 83.

def sign(int) when int >= 0, do: 1
def sign(int) when int < 0, do: -1
        
def reverse(int) when int >= 0 do
    int
    |> Integer.to_string()
    |> String.reverse()
    |> String.to_integer()
end
    
def reset_sign(int, 1), do: int
def reset_sign(int, -1), do: int * -1

def reverse_integer(int) do
    int
    |> abs()
    |> reverse()
    |> reset_sign(sign(int))
end