I think you are just missing an else part. If you don't put the else condition all other transaction type Amount will return null except 'Credit'.
If [Transaction Type] = "Credit" then Amount * -1 else Amount END
As Indumon said, you need to complete your formula. For intance I created a calculated field called Balance with following formula:
Than I used the new measure Balance to calculate the total expenses adding it to Text.
Take a look at the attached workbook.
calculating_balance.twbx.zip 5.4 KB