how do I Make sure the program allows the user to enter the money more than just twice and return the change.

//dispenserType Header File

class dispenserType
{
public:
    int getNoOfItems() const; 
      //Function to show the number of items in the machine.
      //Postcondition: The value of numberOfItems is returned.

    int getCost() const; 
      //Function to show the cost of the item.
      //Postcondition: The value of cost is returned.

    void makeSale();  
      //Function to reduce the number of items by 1.
      //Postcondition: numberOfItems--;

    dispenserType(int setNoOfItems = 50, int setCost = 50); 
      //Constructor 
      //Sets the cost and number of items in the dispenser
      //to the values specified by the user.
      //Postcondition: numberOfItems = setNoOfItems;
      //               cost = setCost;
      //               If no value is specified for a 
      //               parameter, then its default value is 
      //               assigned to the corresponding member 
      //               variable.

private:
    int numberOfItems;     //variable to store the number of 
                           //items in the dispenser
    int cost;  //variable to store the cost of an item
};
//


//cashRegister Header File

class cashRegister
{
public:
    int getCurrentBalance() const;
       //Function to show the current amount in the cash 
       //register.
       //Postcondition: The value of cashOnHand is returned.

    void acceptAmount(int amountIn);
       //Function to receive the amount deposited by 
       //the customer and update the amount in the register.
       //Postcondition: cashOnHand = cashOnHand + amountIn;

    cashRegister(int cashIn = 500); 
       //Constructor
       //Sets the cash in the register to a specific amount.
       //Postcondition: cashOnHand = cashIn;
       //               If no value is specified when the 
       //               object is declared, the default value 
       //               assigned to cashOnHand is 500.

private:
     int cashOnHand;     //variable to store the cash 
                         //in the register
};


// Main Program
#include <iostream>
#include "cashRegister.h"
#include "dispenserType.h"

using namespace std;

void showSelection();
void sellProduct(dispenserType& product, 
                 cashRegister& pCounter);

int main()
{
    cashRegister counter;
    dispenserType candy(100, 50); 
    dispenserType chips(100, 65);
    dispenserType gum(75, 45);
    dispenserType cookies(100, 85);

    int choice;  //variable to hold the selection

    showSelection();
    cin >> choice;

    while (choice != 9)
    {
        switch (choice)
        {
        case 1: 
            sellProduct(candy, counter);
            break;
        case 2: 
            sellProduct(chips, counter);
            break;
        case 3: 
            sellProduct(gum, counter);
            break;
        case 4: 
            sellProduct(cookies, counter);
            break;
        default: 
            cout << "Invalid selection." << endl;
        }//end switch

        showSelection();
        cin >> choice;
    }//end while

    return 0;
}//end main

void showSelection()
{
    cout << "*** Welcome to Shelly's Candy Shop ***" << endl;
    cout << "To select an item, enter " << endl;
    cout << "1 for Candy" << endl;
    cout << "2 for Chips" << endl;
    cout << "3 for Gum" << endl;
    cout << "4 for Cookies" << endl;
    cout << "9 to exit" << endl;
}//end showSelection

void sellProduct(dispenserType& product, 
                 cashRegister& pCounter)
{
    int amount;  //variable to hold the amount entered
    int amount2; //variable to hold the extra amount needed

    if (product.getNoOfItems() > 0) //if the dispenser is not 
                                    //empty
    {
        cout << "Please deposit " << product.getCost()
             << " cents" << endl;
        cin >> amount;

        if (amount < product.getCost())
        {
            cout << "Please deposit another "
                 << product.getCost()- amount
                 << " cents" << endl;
            cin >> amount2;
            amount = amount + amount2;
       }

        if (amount >= product.getCost())
        {
            pCounter.acceptAmount(amount);
            product.makeSale();
            cout << "Collect your item at the bottom and "
                 << "enjoy." << endl;
        }
        else
            cout << "The amount is not enough. " 
                 << "Collect what you deposited." << endl;

        cout << "*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*"
             << endl << endl;
    }
    else
        cout << "Sorry, this item is sold out." << endl;
}//end sellProduct

//Implementation file dispenserType.cpp 
//This file contains the definitions of the functions to 
//implement the operations of the class dispenserType 

#include <iostream>
#include "dispenserType.h"

using namespace std;

int dispenserType::getNoOfItems() const
{
    return numberOfItems;
}

int dispenserType::getCost() const
{
    return cost;
}

void dispenserType::makeSale()
{
    numberOfItems--;
}

dispenserType::dispenserType(int setNoOfItems, int setCost)
{
    if (setNoOfItems >= 0)
         numberOfItems = setNoOfItems;
    else    
        numberOfItems = 50;

    if (setCost >= 0)
        cost = setCost;
    else
        cost = 50;
}


//Implementation file cashRegisterImp.cpp 
//This file contains the definitions of the functions to 
//implement the operations of the class cashRegister

#include <iostream>
#include "cashRegister.h"

using namespace std;

int cashRegister::getCurrentBalance() const
{
    return cashOnHand;
}

void cashRegister::acceptAmount(int amountIn)
{
    cashOnHand = cashOnHand + amountIn;
}

cashRegister::cashRegister(int cashIn)
{
    if (cashIn >= 0)
        cashOnHand = cashIn;
    else
        cashOnHand = 500;
}