Validating -ve/+ve value in the textfield depending on selected radio button

In my jsp page i had One TextField (Say: Amount) and a drop down Box (Say: Months) and two radio buttons (Say 1. Negative 2. Positive)

STEP: 1

By default the "Amount Field is displayed by "0" and dropdown is selected to "Some month" and "Negative" radio button is selected.

I want to validate in such a way that
If i edit the "Amount textfield" with negative value (i.e., Ex: -273) and tabbing out it should not throw an error (Ex: "Invalid data")
because the "Negative" radio button is already clicked. ( see above mentioned STEP:1)

but when u click "Positive" radio button it should through error msg "Invalid data" and also the "Drop down" what is selected should be "disabled"
and the cursor should focus on the "AMount" textfield

If i enter "Positive value" in "AMount field" (Ex: 233) when "Positive radio button" is selected it shouldn't through error msg
if i select "Negative Radio button" then "Drop down" should be enabled and the cursor should point to the "Amount Field".

Note: the values should not be cleared in the "Amount textfield" in any of the scenarios
it should not take any invalid special characters say "@#$!*&^" etc.,

can any body suggest me to implement by giving example