Set values in pandas DataFrame group

How can I assign values in a DataFrame group without triggering SettingWithCopyWarning?

I want to be able to get a reference to a DataFrame group and then be able to access and set column values in that group.

I can’t use .apply or .transform because I will need to reference the preceding group while setting values.

So, for example with:

df = pd.DataFrame(data=(('A', '20210101', 9.0, False),
                        ('B', '20210101', 5.0, False),
                        ('C', '20210101', 5.0, True),
                        ('A', '20210102', 0.0, False),
                        ('B', '20210102', 0.0, False),
                        ('C', '20210102', 0.0, False)),
                  columns=('Name', 'Date', 'Dollars', 'HaveMax')).set_index(('Name', 'Date'))

Suppose I want to set the value of Dollars where Date == '20210102? I have found:

df.groupby('Date').indices.get('20210102')  # << Gets array of the rows
df.iloc(df.groupby('Date').indices.get('20210102'))  # << Gets the rows

But df.iloc(df.groupby('Date').indices.get('20210101'))('Dollars') = 3 gives the SettingWithCopyWarning.

What I think would accomplish my goal would be a code sample that sets each Name’s Dollars on '20210102' to, say, double each Name’s Dollars on '20210101'.