Cancel a PENDING financial transaction (an PAID transaction must be reversed first). The request body is OPTIONAL: the optional 'scope' field (THIS_ONLY default when the body is absent/null) controls what is cancelled: THIS_ONLY only the target; ALL_PENDING every PENDING transaction of the title; THIS_AND_FUTURE the target plus PENDING siblings whose dueDate is on or after the target's dueDate. The title's amountPaid/balance/totalAmount are not changed; when every transaction of the title becomes CANCELLED the title is cancelled as well.