I have already implemented:

iex> Money.Financial.
future_value/2               future_value/3               
interest_rate/3              internal_rate_of_return/1    
net_present_value/2          net_present_value/3          
net_present_value/4          payment/3                    
periods/3                    present_value/2              
present_value/3 

I’ll check against what you require.

Well, I found a way that works. I’m going to share it here in case someone needs it. What I did was to convert the Money types into Decimal types to perform the arithmetic operations well. And then convert to Money again. If someone has a different opinion on what is best way, feel free to discuss

def calculate_annual_amortization(changeset) do
    amount = get_change(changeset, :amount)
    dedicated_percentage = get_change(changeset, :dedicated_percentage)
    initial_value = get_change(changeset, :initial_value) |> Money.to_decimal()
    depreciation_rate = get_change(changeset, :depreciation_rate)

    result =
      initial_value
      |> Decimal.mult(amount)
      |> Decimal.mult(dedicated_percentage)
      |> Decimal.mult(depreciation_rate)
      |> Decimal.div(10_000)
      |> Money.new(:CUP)

    put_change(changeset, :annual_amortization, result)
  end

This does the job and assigns the value correctly.

Not particularly anything on “best way” but just another idea, you were already using with on the initial sample so you could use it to pattern match the result of the Money ops. Or if Money has a ! operator available then you could use that instead to return unwrapped values.

def calculate_annual_amortization(changeset) do
  with(
    amount <- get_change(changeset, :amount),
    dedicated_percentage <- get_change(changeset, :dedicated_percentage),
    initial_value <- get_change(changeset, :initial_value),
    depreciation_rate <- get_change(changeset, :depreciation_rate),
    {:ok, money_1} <- Money.mult(initial_value, amount),
    {:ok, money_2} <- Money.mult(money_1, dedicated_percentage),
    {:ok, money_3} <- Money.mult(money_2, depreciation_rate),
    {:ok, annual_amortization} <- Money.div(money_3, 10_000)
  ) do
    put_change(changeset, :annual_amortization, annual_amortization)

  else
    _ ->
      add_error(changeset, :annual_amortization, "unable to calc annual_amortization")
  end
end

I’m assuming that the fields you’re fetching with get_change are always changed at this point, but if not or you didn’t consider it you might want to change them to fetch_field which always gives back the value (in order #1 the {:changes, value} if changed on the changeset, #2 {:data, existing_value} if unchanged on the changeset, and #3 :error if the field doesn’t exist)
On the with you could then use {_, value} <- fetch_field(changeset, :amount).