(1) The currently applicable amount of the main obligation determines the duty of the surety. This applies in particular also if the main obligation has been changed due to the fault of or default by the principal debtor. The duty of the surety is not extended by a legal transaction that the principal debtor undertakes after assumption of the suretyship.
(2) The surety is liable for the costs of termination and prosecution of rights that are reimbursable by the principal debtor to the creditor.