create one formula called "credits"
if {SRS_FDU.FDU_CORD}='C' then {SRS_FDU.FDU_AMNT}
place it on the detail section.
when you preview the report you should see the actual credit amount on every row that is a credit and a zero on all debit rows.
to get totals for just credits, click on the sigma sign (the blue E in the toolbar) to insert a summary.
select the "Credits" formual field
calculate as a SUM
Summary location will be group 1 (customer)
this will create a field in the group footer that is the sum of credits for each customer.
If you want a sum for the whole report repeat teh last step put for the summary location select Grand Total (report footer) as the location.
Repeat the process for "Debits" using
if {SRS_FDU.FDU_CORD}='D' then {SRS_FDU.FDU_AMNT}
finally create the "Together" using
if {SRS_FDU.FDU_CORD}='C' then {SRS_FDU.FDU_AMNT} else {SRS_FDU.FDU_AMNT} *(-1)
when you place this on the detail row you should see the amount on eery row but if it is a credit is is a positive value if it is a debit it is a negative value.