提供个思路，欢迎其他大佬指正
注意使用 排序（seq），group by，union all

SELECT
	f.* 
FROM
	(
	SELECT
		cus_id,
		max( cusname ) cusname,
		NULL dodate,
		sum( money ) sumMoney,
		NULL payed,
		NULL unpayed,
		1 seq 
	FROM
		tb_outbase 
	GROUP BY
		cus_id UNION ALL
	SELECT
		cus_id,
		cusname cusname,
		dodate dodate,
		NULL sumMoney,
		pay_money payed,
		NULL unpayed,
		2 seq 
	FROM
		tb_paycollect UNION ALL
	SELECT
		cus_id,
		'小计' cusname,
		NULL dodate,
		sum( money ) sumMoney,(
		SELECT
			IFNULL( sum( pay_money ), 0 ) 
		FROM
			tb_pay 
		WHERE
			cus_id = tb_outbase.cus_id 
		) payed,
		sum( money )-(
		SELECT
			IFNULL( sum( pay_money ), 0 ) 
		FROM
			tb_pay 
		WHERE
			cus_id = tb_outbase.cus_id 
		) unpayed,
		3 seq 
	FROM
		tb_outbase 
	GROUP BY
		cus_id 
	) f 
ORDER BY
	f.cus_id,
	f.seq,
	f.dodate