Nice!

Expected and actual values are flipped.

What if there are multiple possible answers?

Edit: Even the first test case has multiple possible answers, and yet there are no specifications on which one should be returned.

This solution modifies the original list.

When I encountered this puzzle, I suspected the goal was to find a cycle in a linked-list. This problem involves graphs -- which is definitely more challenging.
I think you should strongly consider adding "graph" either in the title, the description or both.

I'm edit initial solution

namespace Bank {
  using System;
  public class Account
  {
    private decimal balance;
    public void Deposit(decimal amount)
    {
    }

    public void Withdraw(decimal amount)
    {
    }

    public void TransferFunds(Account destination, decimal amount)
    {
      
    }

    public decimal Balance { get { return balance; } }
  }
}

These seem to have nothing to do with Kata?

When12345_6789ToPoundsReturnsPound12345__68
String lengths are both 9. Strings differ at index 0.
Expected: "?12345.68"
But was: "?12345.68"
WTF?

ShouldParseFloatNumbers
Should parse float numbers
Expected: 3.5d +/- 0.0d
But was: 2.0d
at CalcTest.ShouldParseFloatNumbers () [0x00000] in :0
ShouldParseNumbers
Should parse numbers
Expected: 3.0d +/- 0.0d
But was: 2.0d

WTF?