If you ever need to cancel Contract Lines (Field Service Contract Administration Module Contract Lines that is) outside of GP try this:
DECLARE @RC int
DECLARE @CONSTS smallint
DECLARE @CONTNBR char(11)
DECLARE @LNSEQNBR numeric(19,5)
DECLARE @CANCELDATE datetime
-- TODO: Set parameter values here.
EXECUTE @RC = [dbo].[SVC_Cancel_Contract_Line]
The team that developed the Field Service Series made it quite easy to leverage some of the same stored procedures called by GP to perform various functions such as this.