You should use:
newRow.ToAmount = Decimal.MaxValue;
instead.
You may refer to
money and smallmoney (Transact-SQL)[
^].
Indeed it is stated that money type ranges from -922,337,203,685,477.5808 to 922,337,203,685,477.5807.
So your initial statement seems to be correct (i.e. this value is included in the valid range for a money type).