WITH t AS
(
SELECT 'a' payno, 1 prd, 1000 amt, 1500 totpaid
UNION ALL SELECT 'a',  2, 2000,  1500
UNION ALL SELECT 'a',  3, 7000,  1500
UNION ALL SELECT 'b', 42, 5000, 10000
UNION ALL SELECT 'b', 68, 3000, 10000
)
SELECT payno, prd, amt, totpaid
     , CASE WHEN totpaid > s       THEN amt
            WHEN totpaid < s - amt THEN 0
            ELSE totpaid - s + amt
        END paid
     , CASE WHEN totpaid > s       THEN 0
            WHEN totpaid < s - amt THEN amt
            ELSE s - totpaid
        END left_amt
     , CASE WHEN totpaid > s       THEN totpaid - s
            ELSE 0
        END left_paid
  FROM (SELECT payno, prd, amt, totpaid
             , SUM(amt) OVER(PARTITION BY payno ORDER BY prd) s
          FROM t
        ) a
;