How do I strip dollar signs from a value before validation in Rails?

Three problems here:

As pointed out by Stefan in his answer, you may want to remove the , in your tr! call, though it won't affect the replacement of a $.
You're using tr!, and are using its return value in an incorrect way. tr! (along with most of Ruby's ! method variants) mutates the string in-place and returns nil if no changes were made. Since nil.to_f is 0.0, that's why you're getting that (or maybe not, see below). You should instead use tr.
Rails automatically converts assignment arguments to the correct type for the database column associated with it, so even before validation your value is being converted to a float, and "$400".to_f is 0.0, and that's what your callback sees. The solution is to override amount_due= instead of using a callback:
```
def amount_due=(value)
  value = value.to_s.tr('$', '').to_f
  write_attribute(:amount_due, value)
end
```

There's a comma after $ so you're removing $, instead of $.

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