For payu money, they have defined parameters with defined input names.
Replace your name="textarea" to name="amount" and try.
Let me know still your facing the issue.
Updated Answer
$posted['key'] = $MERCHANT_KEY;
$posted['txnid'] = $txnid;
$posted['amount'] = 100;
$posted['firstname'] = 'Bhushan';
$posted['lastname'] = Lunkad;
$posted['email'] = 'Your email';
$posted['phone'] = 'mobile';
$posted['productinfo'] = 'Product info";
`` $posted['surl'] = 'success url';
$posted['furl'] = 'failure url';
$hashSequence = "key|txnid|amount|productinfo|firstname|email|udf1|udf2|udf3|udf4|udf5|udf6|udf7|udf8|udf9|udf10";
$hashVarsSeq = explode('|', $hashSequence);
$hash_string = '';
foreach ($hashVarsSeq as $hash_var)
{
$hash_string .= isset($posted[$hash_var]) ? $posted[$hash_var] : '';
$hash_string .= '|';
}
$hash_string .= $SALT;
$hash = strtolower(hash('sha512', $hash_string));
Hope it helps you.