SUM return zero in SQL on a string datatype column

I have a column that stores an amount as a varchar. However, I am converting it on below query to decimal as below:

select top 100 cast(amount as decimal(16,4)) amount, *from src.bc_sales_invoice_linewhere document_no = '23005986'

But I need to get the sum of the column and tried on below query:

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But it is returning zero

I need to get the sum of amount on the varchar column. any helps?