No can do. How do you expect to sum 145- $4358.14? In the first place, why is amount of type nvarchar? It defies common sense. Every field in a database table must have the correct data type that befits the purpose, interpretation, and usage of its data.
Change it to the correct data type that it is meant to be by referring to SQL General Data Types
], otherwise you will encounter absurd situation such as this that results in awkward SQL manipulation which is very inefficient and unnecessary.
If the data type is correct, you can get what you want easily:
SELECT name, SUM(amount) FROM tablename GROUP BY name