How to convert varchar into decimal and its sum
The issue in the above query is only due to these value (7,956.00 and 5,328.00 ) when casting in implemented on these values the value returned as 7.00 and 5.00 due to which the query gives the result as 2569.00.

Just remove these commas from these numbers and the query will execute properly

SELECT SUM(REPLACE(Amount, ',', '')) as result 
FROM tblserviceinvoice