If the "amount" is a numeric variable as shown in your data step you cannot have a value of "N/A". You can have a missing value. If you want to see a displayed value of "N/A" then you can create format that will do such but the value can't be text.
This makes a set that looks like that when the format MYNA is available. You would have to make sure the format code is run in any session you want to see N/A, or learn about format management to create and use formats stored in permanent libraries.
data have;
input
name : $1.
id : 8.
code : $8.
amount : 8.
;
datalines;
A 10 12345678 100
A 10 09876543 30
B 9 88997700 20
B 9 88997700 -20
B 9 88997700 40
C 28 11223344 30
C 28 11223344 40
C 28 67676767 50
D 10 78654890 40
D 10 78654890 50
;
proc format;
value myna
.N='N/A'
;
run;
data want;
set have;
by name id;
length firstcode $ 8;
retain firstcode;
if first.id then firstcode=code;
if last.id and code=firstcode then do;
/* write the current values to the data set*/
output;
/* assign missing to amount*/
amount = .N;
output;
end;
else output;
format amount myna. ;
drop firstcode;
run;
SAS has 27 "special" missing values from .A to .Z plus ._ in addition to the default . missing. You can assign formats to display different meanings for each of them for a single variable. The special missing values will appear as A to Z or _ if a format is not assigned to display them differently.
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