Update completion handler outside declared method

Question

I have been trying various solutions to get an updated value of handler from methodTwo and methodThree. The code starts with methodOne(), followed by methodTwo(). How to approach this puzzle?

These methods are declared inside a static library. And its reference implementation i.e. a SampleApp.xcodeproj is calling methodOne(), which is exposed in the header files of the static library.

I have looked into KVO and NSNotificationCenter, but it seems that the direction is not correct.

-(void)methodOne: completion:(void (^)(BOOL success))handler {
    // Step 1
    // Open view controller from PayPal-iOS-SDK
    // BOOL = handlerStatus;

    if (handler != nil)
        handler(handlerStatus);
}

-(void)methodTwo {
    // Step 2   
    // Delegate method from PayPal-iOS-SDK (executed after methodOne())
    // Generates a code which needs to be sent to server to get success/fail
    // Call methodThree to update handlerStatus 
}

-(NSString*) methodThree {
    // Step 3
    // Returns success/fail i.e. handler value
    // Network call (sending code to server)
}

Update

I wanted to use a completion handler, since that's not working. I have to resolve this issue by using delegation as a messaging protocol.