-(int) registerUser
{
self.responseValue = 000; //Notice I set this to 000 at start of method

AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
manager.requestSerializer = [AFJSONRequestSerializer serializer];

NSDictionary *parameters = @{ @"Username": @"SomeUsername" };

[manager POST:@"http://XXX/register"
   parameters:parameters
      success:^(AFHTTPRequestOperation *operation, id responseObject)
 {
     NSLog(@"JSON: %@", responseObject);
     NSError *err = nil;

     self.responseValue = [[responseObject objectForKey:@"res"] intValue];
     //Note: This^ value returns 99 and NSLogs confirm this
 }
      failure:^(AFHTTPRequestOperation *operation, NSError *err)
 {
     NSLog(@"Error: %@", err);
 }];

return self.responseValue; //This will return 000 and never 99!
}

- (void)registerUserWithCompletion:(void (^)(int responseValue, NSError *error))completion
{
    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
    manager.requestSerializer = [AFJSONRequestSerializer serializer];

    NSDictionary *parameters = @{ @"Username": @"SomeUsername" };

    [manager POST:@"http://XXX/register"
       parameters:parameters
          success:^(AFHTTPRequestOperation *operation, id responseObject)
     {
         NSLog(@"JSON: %@", responseObject);

         int responseValue = [[responseObject objectForKey:@"res"] intValue];

         if (completion)
             completion(responseValue, nil);
     }
          failure:^(AFHTTPRequestOperation *operation, NSError *err)
     {
          NSLog(@"Error: %@", err);

          if (completion)
             completion(-1, err); // I don't know what you want to return if it failed, but handle it appropriately
    }];
}

[self registerUserWithCompletion:^(int responseValue, NSError *error) {
    if (error)
        NSLog(@"%s: registerUserWithCompletion error: %@", __FUNCTION__, error);
    else
        NSLog(@"%d", responseValue);

    // do whatever you want with that responseValue here, inside the block
}];

// Needless to say, don't try to use the `responseValue` here, because
// `registerUserWithCompletion` runs asynchronously, and you will probably
// hit this line of code well before the above block is executed. Anything
// that is dependent upon the registration must called from within the above
// completion block, not here after the block.