The error message occurs since you are not using the ComboBox value for the searchstaffsalarypaymentDetails method, but the ComboBox itself.
Maybe you should rewrite it as:
person5.searchstaffsalarypaymentDetails(cboStaffid5.SelectedItem.ToString(), cboPurpose.SelectedItem.ToString(), cboMonth.SelectedItem.ToString(), cboYear.SelectedItem.ToString()))