How to validate just the value in a key_value pair?

I've added a key_value field for "Additional costs" to a custom post type. The key is a text description of the cost and the value is a dollar amount. I'd like to validate submitted keys and values separately.

Selecting validation (for example "Decimals") in the Advanced tab of the Edit Field Group page will validate both the key and value. So it fails because the keys are text.

Questions:

1) Is there any way to validate the submitted key and value separately?
2) Is it possible to validate just the value submitted?

Thanks so much!