Unfortunately I was not able to open the MasterReport in your example so I could not see what you were demonstrating. The MySubReport opened ok, but not the MasterReport. My environment is: VS2005, Telerik 2010 Q2 and VB.NET.
I was able to convert your C# to VB for the textbox ItemDataBound event thus:
procTextBox.Value = procSubReportTextBox.Value
Now when I preview my report I get the following error at the location of TextBox28:
An error has occurred while processing TextBox 'TextBox28':
Index was outside the bounds of the array.
This happens with TextBox28 in the master report footer section and the details section - although it makes no sense to have TextBox28 in the footer because each row in the details section will have its own value for TextBox3, so I have a second problem which is to display in the master report footer section the SUM() of a programmatically assigned textbox in the master report details section. Maybe your example report is helpful in this respect, but sadly I cannot see it.