SQL Group by column AND by date
You just need to use commas instead of AND:

SELECT payment_method, 
       SUM(net_amount),
       DATEADD(MONTH, DATEDIFF(MONTH, 0, transaction_time),0) AS OrderDateByMonth
FROM [Database]
WHERE [criteria]
GROUP BY payment_method,     -- &lt;--- comma here
         DATEADD(MONTH, DATEDIFF(MONTH, 0, transaction_time),0)
ORDER BY payment_method,     -- &lt;--- and here
         OrderDateByMonth ;