Power Pivot :: Calculating Exchange Rates Depending On MONTH

I have created a spreadsheet using powerquery and powerpivot. Loading the data on powerquery from different spreadsheets was a charm, the problem is calculating the exchange rates per month.The structure is as following:I have 3 tables, they are:

Dates– Just a dates (loaded directly on PowerPivot).
Invoice – Details of invoices for all customers, gross totals are in different rates (loaded through power query)
Customer – Detailed information of customers (loaded directly on PowerPivot).
Currency – exchange rates (loaded directly on PowerPivot).

Note: I mentioned where they were loaded because I do not know if that really matters, I guess it does not... but extra information will not hurt. Now, let's go to how these tables are connected between each other and their key columns.The Invoice table has information on invoices dates, invoice gross total and customer code. This table is related to Customer table, which has the actual name of customers and other details. This link is done through the Customer Code column. The Currency table have a relationship with the Customer Table through the Country Column.

What I want to achieve is a formula to do the exchange rate based on the Invoice Date column present on the Invoice Table. So I have created a column on the Currency Table named at each month of the year and populated it with the exchange rates for all countries (countries listed using ISO3 format).My problem is while I am able to select a column from the Currency table to do the exchange rate calculation using the below formula, I fail to realize how to do it per month:=CALCULATE(sum(invoice[Gross Value])*SUM(Currency[Reverse]))*Reverse is the column with the exchange rate for the country money to USD.What I am failing to do is how to make that calculation to also consider the month of the invoice and pull the exchange rate for the same month from the Currency table

Get the invoice date then find the column on Currency[Month] and multiply, providing me the USD amount.I was unable to figure out a solution using the filters parameters from Calculate. If some of you ask why I am using SUM inside Calculate, is because if I only use SUM and try to do the math between the two tables I get an error saying that it was not possible to determine the value on Currency[Reverse]. Same happens with SUMX. It worked out with Calculate, but then I cannot create other formulas using CALCULATE on that table because it will give that error on redundancy, unless I add a filter on it, which worked last time (but I do not remember which thing /parameter used to filter, so do not ask it right now.I do not really need that level of accuracy, but my Virgo perfectionist side demands me to find a solution to that, that will not use a single exchange rate, but consider which is the correct exchange rate for the month of the invoice and calculate it correctly for all invoices. Just so you know, the Invoices tables has more than 1million lines.