public class BillingCalloutService {

@future(callout=true)
public static void callBillingService(String projectRef, Decimal billingAmount){

    ServiceCredentials__c servCred = ServiceCredentials__c.getValues('BillingServiceCredential');

    BillingServiceProxy.project project = new BillingServiceProxy.project();
    project.username = servCred.Username__c;
    project.password = servCred.Password__c;
    project.projectRef = projectRef;
    project.billAmount = billingAmount;

    BillingServiceProxy.InvoicesPortSoap11 invoiceCall = new BillingServiceProxy.InvoicesPortSoap11();
    String response = invoiceCall.billProject(project);

    List<Project__c> proj;
    if (response != null && response.equalsIgnoreCase('OK')){
        proj = [SELECT Status__c FROM Project__c WHERE ProjectRef__c =: projectRef];
        if(proj.size() > 0){
            proj[0].Status__c = 'Billed';
        }
        update proj;
    }


}
}

@RestResource(urlMapping='/project/*')
global with sharing class ProjectRESTService {
@HttpPost
global static String postProjectData(String ProjectRef, String ProjectName,        String OpportunityId, Date StartDate, Date EndDate, Double Amount, String   Status) {
  List<Project__c> projects = [SELECT Billable_Amount__c,End_Date__c,Id,Name,Opportunity__c,ProjectRef__c,Start_Date__c,Status__c FROM Project__c WHERE ProjectRef__c = :ProjectRef];
  Project__c project = new Project__c();
  if(projects.size() > 0)project = projects.get(0);

  Savepoint sp = Database.setSavepoint();
  try{
      project.Billable_Amount__c = Amount;
      project.End_Date__c = EndDate;
      project.Name = ProjectName;
      project.Opportunity__c = OpportunityId;
      project.ProjectRef__c = ProjectRef;
      project.Start_Date__c = StartDate;
      project.Status__c = Status;
      upsert project;

      //Update Opportunity
      Opportunity opportunity = [SELECT Id, DeliveryInstallationStatus__c FROM Opportunity WHERE Id = :OpportunityId];
      opportunity.DeliveryInstallationStatus__c = 'In progress';
      update opportunity;
      If(Test.isRunningTest()){Integer i=10/0; return 'OK';}return 'OK';
  }
  catch(exception ex){
      Database.rollback(sp);
      return ex.getMessage();
  }
}
}