Try something like this;

put one hidden button like

<asp:TextBox ID="txtchange" runat="server" AutoPostBack="true" onchange="javascript:onchange();" OnTextChanged="txtchange_OnChange"></asp:TextBox>
<input type="button" runat="server" id="hidbtn" style="display:none;" onserverclick="txtchange_OnChange" />

You can raise the same textchage event in button serverclick.

In clientside do this

function onchange()
    {     
         if(parseInt(value) > 100)
         {
                  alert('Amount is greater than 100!');
                  return false;           
         }
        else
         {
                  document.getElementById('hidbtn').click();
                  return true;
        }

    }

May be this will help you.

Or you can use __doPostBack(textBox.ID,null); also

Best Luck!!!!!!!!!!!!
Sujit.