You want to try out this?

SELECT
  Columna,
  Columnb,
  Columnc,
  CASE
    WHEN (SELECT SUM(*) FROM YOURTABLE WHERE Columnb between 1 and Columnb - 1) < 6000 THEN Columnc
    ELSE (SELECT 6000 - SUM(*) FROM YOURTABLE WHERE Columnb between 1 and Columnb - 1)
  END AS Columnd
FROM YOURTABLE;