To save with variable in save() function

I'm a little unclear still what you're trying to accomplish, but with() variables are NOT meant to be saved into directly - they are meant only to automatically evaluate based on changes (if applicable) in the data in their original definitions. Hence why you're getting the error message which is expected.

Then in my case how will i achieve my goal.
I need to compare data dynamically. I mean first time if i select anything in with than i am preparing this dataset which i am passing to another rule under grid layout(Under new with) where i am comparing two field values , but my validation is working with existing values in dataset, but when i enter new value in both fields which is not available in that dataset, than i am getting this error as i am trying to save this value in this dataset.

Then in my case how will i achieve my goal.
I need to compare data dynamically. I mean first time if i select anything in with than i am preparing this dataset which i am passing to another rule under grid layout(Under new with) where i am comparing two field values , but my validation is working with existing values in dataset, but when i enter new value in both fields which is not available in that dataset, than i am getting this error as i am trying to save this value in this dataset.

Then in my case how will i achieve my goal.
I need to compare data dynamically. I mean first time if i select anything in with than i am preparing this dataset which i am passing to another rule under grid layout(Under new with) where i am comparing two field values , but my validation is working with existing values in dataset, but when i enter new value in both fields which is not available in that dataset, than i am getting this error as i am trying to save this value in this dataset.

I'm not clear what exactly you're trying to do - can you start out by explaining your use case at a more high level?

It looks like you're trying to directly edit a data set, and in that case the usual method is to query the original data in-process, and pass it into your SAIL form via a rule input (ri!) variable. Your editable grid is then free to save changed values into the ri! variable, which in this case acts similar to a load() variable. You would then save any changes back to the database in your process model after the form is submitted.

(Also: in the case where your reply accidentally gets submitted multiple times, you can click the "More" link under the comment body and "Delete" on the extra copies.)

Also, just as a suggestion, where you do this:

saveInto: {
  ri!masterDataSet[ri!index].nameCopy,
  a!save(
    ri!masterDataSet[ri!index].nameCopy,
    upper(
      ri!masterDataSet[ri!index].nameCopy
    )
  )
}

You could replace this with simply:

saveInto: {
  a!save(
    ri!masterDataSet[ri!index].nameCopy,
    upper(save!value)
  )
}