By default leave the href attribute in the tag empty and give it a id
<input type='text' name='payment' onblur="add_value(this.value);">
<a id='pay_button'>Pay</a>
<script type='text/javascript'>
function add_value(value)
{
var url = 'https://www.payfast.co.za/eng/process?md=_paynow&receiver=support%40payfast.co.za&?item_name=Black+Eye+Boxing+Gloves&amount='+value;
var elem = document.getElementById('pay_button');
elem.setAttribute("href", url);
}
</script>