You can use Commandname property like this for button of nested repeater:
<asp:Repeater ID="rprSSNested" runat="server" OnItemCommand="rprSSNested_ItemCommand" >
<HeaderTemplate>
</HeaderTemplate>
<ItemTemplate>
//******Some Items******
</ItemTemplate>
<FooterTemplate>
<div style=" padding: 20px 35px;" id='ajax'>
<asp:TextBox ID="textbox" TextMode="MultiLine" Columns="50" Rows="10" runat="server" ></asp:TextBox>
<br />
<asp:Button ID="Button2" runat="server" OnClick="Button2_Click" Text="Save" CommandName="cmd" CommandArgument="arg"/>
</div>
</FooterTemplate>
</asp:Repeater>
And add event in c# code like this:
protected void rprSSNested_ItemCommand(object source, RepeaterCommandEventArgs e)
{
if (e.Item.ItemType == ListItemType.Footer)
{
if (e.CommandName == "cmd")
{
string ss = ((TextBox)e.Item.FindControl("textbox")).Text;
Response.Write(ss);
}
}
}