Cus_no   Inv_No    Trans_type   Amt_Inv   Amt_Paid
100      12345      Other         0        -21.76
100      12345      Discount      0         -6.39
100      12345      Discount      6.39     6.39
100      12345      Discount      21.76    21.76
100      12345      Sales Inv     218.99   218.99


What I would like to do is a select statement that returns:

Cus_no   Inv_no    Amt_Inv     Amt_Paid  Disc_amt    Other
100      12345      247.14      218.99    -6.39      -21.76    


I've tried something like this but since I'm referencing trans_type I get a message that trans_type must be in Group by. doing that give me multiple records. 

select cus_no, Inv_no, Sum(Amount_Invoiced_DC) AS InvAmt,
case trans_type when 'Sales Inv' then sum(Amount_Paid_DC) else 0 end as AmtPaid,
case when trans_type = 'Discount' and sum(Amount_Paid_DC)<0 then sum(Amount_Paid_DC) else 0 end as DiscountAmt
FROM            BI50_BankTransactions_AR_InvcDt_H
where cus_no is not null 
group by cus_no, Inv_no