The book implements handle_command/2 for :sub as follows:

def handle_command(%{value: val}, %{cmd: :sub, value: v}) do
  %{event_type: :value_subtracted, max(@min_state_value, val - v)
end

The value in the created event is the result expected after subtraction, i.e., it is not the subtrahend. It can be reimplemented as follows instead:

def handle_command(%{value: val}, %{cmd: :sub, value: v) do
  subtrahend = if(val >= v, do: v, else: val)
  %{event_type: :value_subtracted, value: subtrahend}
end