ทำไหมมันผิดได้ขนาดนี้ครับ งงกับปัญหาจริง ช่วยดูหน่อยน่ะครับ

<table width="648" border="0" cellpadding="0" cellspacing="0">
  <!--DWLayoutTable-->
  <tr>
    <td width="648" height="21" align="center" valign="middle" background="../PhotoWeb/bg03.png" class="style37">ข้อมูลการชำระหนี้</td>
    </tr>
  <tr>
    <td height="16"></td>
    </tr>
  <tr>
    <td height="150" valign="top">
   <form id="payform" name="payform" method="post" action="PayAddtemp.php" onsubmit="return checkpay()">
      <table width="100%" border="0" cellpadding="0" cellspacing="0" class="csstable">
        <!--DWLayoutTable-->
        <tr>
          <td width="131" height="30" align="right" valign="middle" class="style2">เลขที่ใบแจ้งหนี้</td>
            <td width="224" align="left" valign="middle" class="style32">&nbsp;&nbsp;<img src="../PhotoWeb/bulet-1.gif" />&nbsp;&nbsp;&nbsp;<?=$iddebt1?>
            <input type="hidden" name="iddebt" value="<?=$iddebt?>" />
            <input type="hidden" name="idmem" value="<?=$idmem?>" /></td>
            <td width="88" align="right" valign="middle" class="style2">ชื่อ - นามสกุล</td>
            <td width="205" align="left" valign="middle" class="style32">&nbsp;&nbsp;<img src="../PhotoWeb/bulet-1.gif" />&nbsp;&nbsp;&nbsp;<?=$namemem?></td>
          </tr>
        <tr>
          <td height="30" align="right" valign="middle" class="style2">วันที่แจ้งหนี้</td>
            <td align="left" valign="middle" class="style32">&nbsp;&nbsp;<img src="../PhotoWeb/bulet-1.gif" />&nbsp;&nbsp;&nbsp;<?="$Dateday$Datemonth$Dateyear"?></td>
            <td rowspan="2" align="right" valign="top" class="style2">ที่อยู่</td>
            <td rowspan="2" align="left" valign="top" class="style32">&nbsp;&nbsp;<img src="../PhotoWeb/bulet-1.gif" />&nbsp;&nbsp;&nbsp;<?=$addressmem?></td>
          </tr>
        <tr>
          <td height="30" align="right" valign="middle" class="style2">ยอดหนี้</td>
            <td align="left" valign="middle" class="style32">&nbsp;&nbsp;<img src="../PhotoWeb/bulet-1.gif" />&nbsp;&nbsp;&nbsp;<?=$sumper?></td>
          </tr>
        <tr>
          <td height="30" align="right" valign="middle" class="style2">ยอดค้างชำระหนี้</td>
            <td align="left" valign="middle" class="style32">&nbsp;&nbsp;<img src="../PhotoWeb/bulet-1.gif" />&nbsp;&nbsp;&nbsp;<?=$remainpay?><input type="hidden" name="remainpay" id="remainpay" value="<?=$remainpay?>" /></td>
            <td align="right" valign="middle" class="style2">เบอร์โทร</td>
            <td align="left" valign="middle" class="style32">&nbsp;&nbsp;<img src="../PhotoWeb/bulet-1.gif" />&nbsp;&nbsp;&nbsp;<?=$telmem?></td>
          </tr>
        <tr>
          <td height="30" align="right" valign="middle" class="style2">จำนวนเงินที่ชำระ</td>
            <td colspan="3" align="left" valign="middle" class="style2">
              <input type="text" name="pay" id="pay" size="10"> 
              / บาท 
              &nbsp;&nbsp;&nbsp;&nbsp;<input type="submit" name="addtemp" id="addtemp" value="ชำระหนี้" /></td>
          </tr>
        </table></form></td>
    </tr>
</table>
<br>
<table width="648" border="0" cellpadding="0" cellspacing="0">
  <!--DWLayoutTable-->
  <tr>
    <td width="648" height="25" align="center" valign="top" class="style39"><a href="MainMember.php?action=PayListdata"><img src="../PhotoWeb/pw25.png" border="0"></a><br>
      ย้อนกลับ</td>
  </tr>
</table>
</center>

<script language="javascript">
document.payform.pay.value=="0";
function checkpay()
{
	if(document.payform.pay.value == "")
	{
		alert('กรุณากรอกจำนวนเงินที่ต้องการจะชำระหนี้ด้วย');
		document.payform.pay.focus();
		return false;
	}
	if(document.payform.pay.value > document.payform.remainpay.value )
	{
		alert(document.payform.pay.value+'>'+document.payform.remainpay.value);
		document.payform.pay.focus();
		return false;
	}
}
</script>