Hey, I came up with 2 solutions so far:

for order <- buy do
      Enum.reduce(
        buy,
        order,
        fn x, y ->
          cond do
            Decimal.cmp(x.price, y.price) == :eq and x.id != y.id ->
              new_amount = Decimal.add(x.amount, y.amount)
              Map.put(y, :amount, new_amount)

            true ->
              y
          end
        end
      )
    end
    |> Enum.uniq_by(fn x -> x.price end)

and

    for order <- buy do
      Enum.map(buy, fn x ->
        cond do
          Decimal.cmp(x.price, order.price) == :eq and x.id != order.id ->
            new_amount = Decimal.add(x.amount, order.amount)
            Map.put(order, :amount, new_amount)

          true ->
            order
        end
      end)
    end
    |> List.flatten()
    |> Enum.uniq_by(fn x -> x.price end)

But I am having a hard time deciding which one to stick with
Edit:
I decided to stick with the reduce version, so I don’t need to flatten a list