'Custom' group by in select

After some more time looking for a solution, I finally got it. I'll post an answer for future users to have access.

Solution:

With Sales As (
   Select
      Id_Product,
      Sum(Qt_Sold) Sold_Amount,
      Sum(Net_Value) Total_Net_Value,
      Case Extract(Month From Date_Sale) || '' || Extract(Year From Date_Sale)
         When Extract(Month From Current_TimeStamp) || '' || Extract(Year From Current_TimeStamp) Then 'MonthA'
         When Extract(Month From DateAdd(Month, -1, Current_TimeStamp)) || '' || Extract(Year From DateAdd(Month, -1, Current_TimeStamp)) Then 'MonthB'
         Else 'MonthC'
      End Month_Of_Sale
   From
      Sale
   Group By
      Id_Product,
      Month_Of_Sale
   Order By
      Id_Product,
      Month_Of_Sale
)
Select
   Id_Product,
   Sum(Sold_Amount) Sold_Amount,
   Sum(Total_Net_Value) Total_Net_Value,
   Min(Month_Of_Sale) Month_Of_Sale
From
   Sales
Group By
   Id_Product
Order By
   Id_Product