TransferTests.cs

    [TestFixture]
    public class TransferTests
    {

        [Test]
        public void AmountTransferredFromPayerToPayee()
        {
            var transfer = new FundsTransfer();
            
            // initialise accounts
            var payerBalance = transfer.InitAccount();
            var payeeBalance = transfer.InitAccount();
            
            // credit payer with funds to allow transfer
            transfer.Credit(ref payerBalance, 100);

            // transfer 50
            transfer.Transfer(ref payerBalance, ref payeeBalance, 50);

            Assert.That(payerBalance, Is.EqualTo(50));
            Assert.That(payeeBalance, Is.EqualTo(50));
        }

    }

    public class FundsTransfer
    {
        public double InitAccount()
        {
            double balance = 0;
            return balance;
        }

        public void Transfer(ref double payerBalance, ref double payeeBalance, double amount)
        {
            Debit(ref payerBalance, amount);
            Credit(ref payeeBalance, amount);
        }
        
        public void Credit(ref double balance, double amount)
        {
            balance += amount;
        }

        public void Debit(ref double balance, double amount)
        {
            balance -= amount;
        }
    }