HiRumeValid,

You need to add else condition to remove the disabled attribute.

Check this example. Now please take its reference and correct your code.

HTML

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script type="text/javascript">
    $(function () {
        $("[id*=drpusing]").change(function () {
            var row = $(this).closest("tr");
            var value = $(this).val();
            if (value == "P") {
                row.find("[id*=txtpayitem]").attr("disabled", "true");
            } else {
                row.find("[id*=txtpayitem]").removeAttr("disabled");
            }
        });
    });
</script>
<div>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:TemplateField HeaderText="S/N">
                <ItemTemplate>
                    <span id="indxid" class="indxno" style="text-align: center">
                        <%# Container.DataItemIndex+1 %>
                    </span>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:BoundField DataField="Name" HeaderText="Name" />
            <asp:TemplateField HeaderText="Using">
                <ItemTemplate>
                    <asp:DropDownList runat="server" ID="drpusing">
                        <asp:ListItem Text="-- Select--"></asp:ListItem>
                        <asp:ListItem Text="Value" Value="V"></asp:ListItem>
                        <asp:ListItem Text="Percentage" Value="P"></asp:ListItem>
                    </asp:DropDownList>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Payitem to use to derive value">
                <ItemTemplate>
                    <asp:TextBox runat="server" ID="txtpayitem"></asp:TextBox>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
</div>

Namespaces

C#

using System.Data;

VB.Net

Imports System.Data

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Name");
        dt.Rows.Add("Mudasser Khan");
        dt.Rows.Add("John Hammod");
        dt.Rows.Add("Rober Mathew");
        this.GridView1.DataSource = dt;
        this.GridView1.DataBind();
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim dt As DataTable = New DataTable()
        dt.Columns.Add("Name")
        dt.Rows.Add("Mudasser Khan")
        dt.Rows.Add("John Hammod")
        dt.Rows.Add("Rober Mathew")
        Me.GridView1.DataSource = dt
        Me.GridView1.DataBind()
    End If
End Sub

Screenshot