There should be no problem to do this without writing code. Quick sidenote: if your clients are using IP2003, make sure you always save your solutions in IP2003 (I know it's obvious but wanted to make sure).
So, to achieve what you're trying to do, it's best to use rules. Double click on a control bound to Actual Amount, select "Rules", click on "Add", give your rule a nice meaningful name, click on "Add Action", select "Set a field's value" from the dropdown, pick your target field (namely "Associated Budget") and in the value box, pick your current Actual Amount node.
Now any time Associated Budget changes, the Actual Amount will change too.
Rules are smart so they will know to updated only the sibling Actual Amount in the repeating context without affecting others. Note that you will have to make sure that initially both fields contain the same value.
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Actually, now that I think of it, there is an even easier way of doing it with default values, aka "calculations". Simply double-click on Actual Amount, click on "Fx" button next to the Value, "Insert Field or Group" and pick Associated Budget. This way you get the same fuctionality without having to worry about the initial value. In general, calculations should always be used before rules since they are simpler to use (but les powerful - you cannot do conditionals likes rules can).
Hope I didn't make this too confusing!
David Airapetyan / Software Services ArchitectQdabra® Software / Streamline data gathering to turn process into knowledge