I need to minus the dollar amount in Table A from Table B when they meet the criteria. Can T-SQL do this function?

For Example, when Year, MonthID, AcNo and Pid in Table A are the same as those in Table B, the Amt in Table A needs to minus the Amt in Table B.

Table A
Year MonthID AcNo Pid Amt
2002 1 1001 1 2000
2002 1 1001 2 2500
2002 1 1001 3 3000

Table B
Year MonthID AcNo Pid Amt
2002 1 1001 1 200
2002 1 1001 2 500
2002 1 1001 3 30

Though I am not clear with your requirement .. I guess Triggers can be of use to you.

Best Regards
Vadivel

MVP ASP/ASP.NET
http://vadivel.thinkingms.com

Rough Guidelines:

For existing data run a query something like:
UPDATE Table1
SET Table1.Amount = Table1.Amount - Table2.Amount
FROM Table1 INNER JOIN Table2
ON Table1.PID = Table2.PID
AND Table1.[Year] = Table2.[Year]
AND Table1.MonthID = Table2.MonthID
AND Table1.AcNo = Table2.AcNo
WHERE Table2.Amount IS NOT NULL

For future use a Trigger something like:
CREATE Trigger After_IU_Update_T1_From_T2
ON Table2 AFTER Insert,Update
AS IF Update(Amount)
    UPDATE Table1
    SET Table1.Amount = Table1.Amount - inserted.Amount
    FROM Table1 INNER JOIN inserted
    ON Table1.PID = inserted.PID
    AND Table1.[Year] = inserted.[Year]
    AND Table1.MonthID = inserted.MonthID
    AND Table1.AcNo = inserted.AcNo
WHERE inserted.Amount IS NOT NULL