So, if I understand you correctly, you're displaying the opening balance that is entered as a parameter and then calculating the closing balance in the report. When you put the closing balance in the group header, it's showing the same value as the opening balance. This happens because the closing balance is calculated while the data is processing so it's generally only available in the footer.
For the sake of this example, I'm going to call the field you're calculating from {MyTable.Amount} and the parameter {?OpeningBalance}. Try creating a formula that looks something like this:
{?OpeningBalance} + Sum({MyTable.Amount}, {MyTable.VoucherDate})
Summary functions are calculated in such a way that they can be used in either the group header or footer. This formula will add up all of the amount values using a summary function and then add it to the opening balance. Using this technique modifed for your data should give you what you're looking for.
-Dell
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