How to automaticaly uncheck a checkbox if the textbox is clicked

I'm building a donation form.

If a checkbox with a fixed amount is checked by the user, I would like it to automatically uncheck if next the user clicks the textbox to insert a custom amount.

I know nothing of JavaScript. Will someone help me achieve this?

<form action="http://ift.tt/rDmnwQ" method="post" target="_blank">

    <input type="hidden" name="cmd" value="_donations">
    <input type="hidden" name="business" value="louzanimalespaypal@gmail.com">

    <input type="checkbox" name="amount" class="checkbutton" value="5.00"><span>€05.00</span>
    <input type="checkbox" name="amount" class="checkbutton" value="5.00"><span>€10.00</span>
    <input type="checkbox" name="amount" class="checkbutton" value="5.00"><span>€15.00</span>
    <input type="checkbox" name="amount" class="checkbutton" value="5.00"><span>€20.00</span>
<br>
    <strong>Other Amount</strong><br>
    $ <input type="text" class="TextBox" name="amount">

    <input type="hidden" name="item_name" value="Donation">
    <input type="hidden" name="item_number" value="Donation">
    <input type="hidden" name="currency_code" value="EUR">
    <input type="hidden" name="lc" value="PT">
    <input type="hidden" name="bn" value="Louzanimales_Donation_WPS_PT">
    <input type="hidden" name="return" value="http://ift.tt/2hQztX7">

    <br /><br />
    <input type="submit" value="Pay with PayPal!">

</form>

.

<script>
    $('input.checkbutton').on('change', function() {
        $('input.checkbutton').not(this).prop('checked', false);  
    });
</script>

Fiddle