How can I help SQL Server recognize my indexed view column is NOT NULL-able?
user_id, currency_id, and transaction_amount are all defined as NOT
  NULL columns in dbo.transactions


It looks to me that SQL Server has a blanket assumption that an aggregate can produce a null even if the field(s) it operates on are not null. This is obviously true in certain cases:

create table foo(bar integer not null);
select sum(bar) from foo
-- returns 1 row with `null` field


And is also true in the generalized versions of group by like cube

This simpler test case illustrates the point that any aggregate is interpreted as being nullable:

CREATE VIEW dbo.balances
with schemabinding
AS
SELECT
      user_id
    , sum(1)   AS balance_amount
FROM dbo.transactions
GROUP BY
      user_id
;
GO


IMO this is a limitation (albeit a minor one) of SQL Server - some other RDBMSs allow the creation of certain constraints on views that are not enforced and exist only to give clues to the optimizer, though I think 'uniqueness' is more likely to help in generating a good query plan than 'nullability'



If the nullability of the column is important, perhaps for use with an ORM, consider wrapping the indexed view in another view that simply guarantees the non-nullability using ISNULL:

CREATE VIEW dbo.balancesORM
WITH SCHEMABINDING
AS
SELECT 
    B.[user_id],
    B.currency_id,
    balance_amount = ISNULL(B.balance_amount, 0),
    transaction_count = ISNULL(B.transaction_count, 0)
FROM dbo.balances AS B;