When a certain data is bound to RadGrid rows, that data will be processed later to calculate aggregate values as well. By changing the Text property of a Cell will not be bound to the Cell nor will be included in the aggregate values. It is like changing a variable name will not change the value assigned to it.
Having that in mind, if you are manually changing the data, you might as well update the aggregate values manually.
I happen to have an example of manipulating the aggregates by calculating only selected rows' values. Check and download the attachment.
In your case, you will need to update the aggregate values similar to the example.
I hope this will prove helpful.
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