Can't always change the amount of an existing transfer

[kbaley]

To reproduce:

1. Enter a transfer
2. Open one side of the transaction for editing
3. Tab from the date to the amount
4. Change the amount
5. Press enter to save

There is an error in the controller because the transaction has an invalid categoryId (Starting with 'TRF:'). Workaround: Click directly into the amount and don't tab through. Either the vendor or category fields are setting the categoryId on the transaction.