Where any surety company bond is required to be executed by any county, district, or state officer or his deputy or by any county employee who has in his custody any money or property belonging to the county for the purposes of his employment, whether said bond is required by law or by the board of county commissioners or by the principal officer, such surety or sureties may, if they deem themselves unsafe or insecure, upon thirty (30) days written notice given to the Secretary of State as to state and district officers and as to county officers the board of county commissioners, withdraw and cancel their obligations as surety or sureties on said bond; provided that such cancellation shall not relieve the surety company from any liability previously incurred, and said pro rata share of the unearned premium shall be returned.