AVG using specific condition for each row
You could do something like this.

Quantity2015 = SUM(CASE WHEN Year = 2015 THEN QuantityTotal ELSE 0 END)
Quantity2016 = SUM(CASE WHEN Year = 2016 THEN QUantityTotal ELSE 0 END)


Not very flexible, but it does work.  You can use other aggregators than SUM as well.  

Ok, so a full query example would look like this.  You might need to correct table names (especially for Client/Product).  

SELECT Q.QuotationId
    , C.ClientName
    , P.ProductName
    , Quantity2015 = SUM(CASE WHEN S.[Year] = 2015 THEN S.Quantity ELSE 0 END)
    , Quantity2016 = SUM(CASE WHEN S.[Year] = 2016 THEN S.Quantity ELSE 0 END)
FROM Quotation AS Q
    INNER JOIN Client AS C ON C.ClientID = Q.ClientID
    INNER JOIN QuotationDetail AS QD ON QD.QuotationID = Q.QuotationID 
    INNER JOIN Product AS P ON P.ProductID = QD.ProductID
    INNER JOIN Sales AS S ON S.ProductID = QD.ProductID AND S.ClientID = Q.ClientID 
GROUP BY Q.QuotationId
    , C.ClientName
    , P.ProductName