Table A:
* Created At
* Amount Charged

Table B:
* Created At
* Cash Amount

SELECT str_to_date(concat(date_format(created_at, '%Y-%m'), '-01'), SUM(sub.c)
FROM (SELECT CreatedAt, Amount AS c
      FROM tabA
      UNION ALL CreatedAt, Cash
      FROM tabB) sub
GROUP BY str_to_date(concat(date_format(created_at, '%Y-%m'), '-01');