Error While Trying To Execute An SP In My CASE STATEMENT

Hi all,
How do i execute a stored procedure in the THEN CLAUSE of my CASE STATEMENT? Av been getting errors since.

Here is my code:



Alter PROCEDURE sp_getTxn (
@m1 int = Null,
@txn int = Null,
@p2 int = Null,
@amt int = Null,
@pAccountno varchar(50) = 'Null',
@DAcct int = Null,
@Balance Decimal(19,4) = NULL OUTPUT,
@pBalance Decimal(19,4) = NULL OUTPUT,
@RowsReturned smallint = NULL OUTPUT )
AS
SET NOCOUNT ON

select CASE
WHEN @m1 = 200 THEN case
when @txn = 00 then ('exec dbo.CustOrderHist (@CrAcct int)')

when @txn = 01 then ('exec dbo.Sp_withdrawal')

when @txn = 31 then exec dbo.CheckBalance(@pAccountno varchar(50), @pBalance Decimal(19,4) OUTPUT)

when @txn = 38 then ('exec dbo.Sp_StatementOfAcct')
END
END
WHEN @m1 = 420 THEN case
when @txnType = 00 then ('exec dbo.Sp_reversal')

when @txnType = 01 then ('exec dbo.Sp_reversal2')

when @txnType = 31 then ('exec dbo.Sp_reversal3')
END
END


SET @Balance = @pBalance
Print @Balance


Or is there an alternative to the above CASE statement that is easier and faster?

Thanks