Sounds like you have a linked cash account setup for that investment account.No cash is held in the investment account with this setup. All cash transactions are in the linked cash account, and the money is transferred using BuyX and SellX.
Are you trying to enter you sale directly into the transaction list and not clicking on "Enter Transaction" and entering the trade using the pop-up window? When you say "Money option in the drop down is not available to be changed" and refer to "SellX" that does suggest a direct entry in the transaction list to me. I just tried a test case and is seemed to work just fine.Can you use the "Enter Transaction" route sucessfully?