Calling Procedure to Update Table

Dear experts,

most likely i'm not seeing the wood for the trees

I created a procedure thats supposed to do some currency conversion for me:

create procedure SCHEMA.CC    (     in SID nvarchar(3),     in MANDT nchar(3),     in DATUM date,     in START_CURRENCY_C nvarchar(5),     in TARGET_CURRENCY_C nvarchar(5),     in START_CURRENCY_V dec(15,2),     in TYPE_OF_RATE char(2) default 'M',     out TARGET_CURRENCY_V dec(15,2)     )     LANGUAGE SQLSCRIPT SQL SECURITY INVOKER AS     
BEGIN 

-- some Variables
DECLARE CT nchar(5);
DECLARE BW nchar(5);
DECLARE XI nchar(1);
DECLARE UKURS dec(9,5);
DECLARE FFACT dec(9,0);
DECLARE TFACT dec(9,0);    
DECLARE DUMMY integer;    


-- Start currency and target currency match?
if TARGET_CURRENCY_C = START_CURRENCY_C
then TARGET_CURRENCY_V := START_CURRENCY_V;
-- then ...

-- #####################
-- some coding here 
-- #####################

-- calculation
            TARGET_CURRENCY_V := START_CURRENCY_V * FFACT / UKURS * TFACT;    end if;
END
;

Then i want to use it for updating a col in a table using something like the following code:

update SCHEMA.TABLE as T
set COL_EUR = (select TARGET_CURRENCY_V from SCHEMA.CC_TMP(T."SID", T."MANDT", T."AUGDT", T."WAERS", 'EUR', T."WRBTR", 'M')) 

Any help to shed some light on this issue would be highly appreciated