How to find right values for API parameters

@Link You should send this task as a background job. There is a template below.

DECLARE
	attr_    VARCHAR2(32000);
	sql_msg_ VARCHAR2(32000);
	stmt_    VARCHAR2(32000);
	job_id_  NUMBER;
BEGIN
	stmt_    := '
BEGIN


END;';
	sql_msg_ := ifsapp.message_sys.construct('UPD');
	ifsapp.message_sys.add_attribute(sql_msg_, 'SQL', stmt_);
	ifsapp.client_sys.clear_attr(attr_);
	ifsapp.client_sys.add_to_attr('SQL_DATA_', sql_msg_, attr_);
	ifsapp.client_sys.add_to_attr('MSG_', '', attr_);
	ifsapp.transaction_sys.deferred_call(job_id_, 'Fnd_Event_Action_API.Action_Executeonlinesql', 'PARAMETER', attr_, 'Geri plan Tanımı');
END;

And for your scenario. You can use it like this.

DECLARE
	attr_    VARCHAR2(32000);
	sql_msg_ VARCHAR2(32000);
	stmt_    VARCHAR2(32000);
	job_id_  NUMBER;
BEGIN
	stmt_    := '
DECLARE
    attr_               VARCHAR2(800);
    v_buy_qty_due       NUMBER(8, 2) := ((''&NEW:BUY_QTY_DUE'' * ''&LENGTH'') / 1000);
    v_customer_quantity NUMBER(8, 2) := ((''&NEW:CUSTOMER_QUANTITY'' * ''&LENGTH'') / 1000);
    v_objversion        VARCHAR2(100);
    v_rowid             VARCHAR2(100);
    info_               VARCHAR2(2000);
BEGIN
    SELECT objid,
           objversion
    INTO   v_rowid,
           v_objversion
    FROM   ifsapp.external_cust_order_line_all
    WHERE  message_id = ''&NEW:MESSAGE_ID''
           AND catalog_no = ''&NEW:CATALOG_NO''
           AND line_no = ''&NEW:LINE_NO'';
    ifsapp.client_sys.clear_attr(attr_);
    ifsapp.client_sys.add_to_attr(name_ => ''BUY_QTY_DUE'', value_ => v_buy_qty_due, attr_ => attr_);
    ifsapp.client_sys.add_to_attr(name_ => ''CUSTOMER_QUANTITY'', value_ => v_customer_quantity, attr_ => attr_);
    ifsapp.external_cust_order_line_api.modify__(info_, v_rowid, v_objversion, attr_, ''DO'');
END;
';
	sql_msg_ := ifsapp.message_sys.construct('UPD');
	ifsapp.message_sys.add_attribute(sql_msg_, 'SQL', stmt_);
	ifsapp.client_sys.clear_attr(attr_);
	ifsapp.client_sys.add_to_attr('SQL_DATA_', sql_msg_, attr_);
	ifsapp.client_sys.add_to_attr('MSG_', '', attr_);
	ifsapp.transaction_sys.deferred_call(job_id_, 'Fnd_Event_Action_API.Action_Executeonlinesql', 'PARAMETER', attr_, 'Geri plan Tanımı');
END;