Resposta - Validação de Saque

Classe AccountSecurity:

namespace exercise82.Models;

internal class AccountSecurity
{
    public bool ValidateWithdrawal(double value)
    {
        return value <= 1000;
    }
}

Classe BankAccount:

namespace exercise82.Models;

internal class BankAccount
{
    private AccountSecurity security;
    public string Holder { get; set; }
    public double Balance { get; private set; }

    public BankAccount(string holder, double balance)
    {
        Holder = holder;
        Balance = balance;
        security = new AccountSecurity();
    }

    public void WithdrawMoney(double value)
    {
        if (!(security.ValidateWithdrawal(value)) || value > Balance)
        {
            Console.WriteLine($"Withdrawal of ${value:F2} denied.");
        }
        else
        {
            Balance -= value;
        }
    }

    public override string ToString()
    {
        return $"Holder: {Holder} - Balance: ${Balance:F2}";
    }
}

Instância:

using exercise82.Models;

BankAccount bankAccount = new BankAccount("Carlos Eduardo", 2000);
bankAccount.WithdrawMoney(1500);
bankAccount.WithdrawMoney(700);

Console.WriteLine(bankAccount);