Here's your DDL and DML adjusted so it doesn't throw an error and doesn't implicitly convert numbers as text into decimal. Run it, check the figures are what you expect, if not then amend and post back:

--===== If the test table already exists, drop it

IF OBJECT_ID('TempDB..#tmpCustomerAmount','U') IS NOT NULL

DROP TABLE #tmpCustomerAmount

CREATE TABLE #tmpCustomerAmount (

[id] [int] IDENTITY(1,1) NOT NULL,

AmountSold [decimal](13, 2) NULL,

AmountLeftWith [decimal](13, 2) NULL

)

SET IDENTITY_INSERT #tmpCustomerAmount ON

iNSERT INTO #tmpCustomerAmount (ID, AmountSold, AmountLeftWith)

SELECT 1,123.00,0 UNION ALL

SELECT 2,130000,0 UNION ALL

SELECT 3,500.00,0 UNION ALL

SELECT 4,700.00,577 UNION ALL

SELECT 5,300.00, 0

SELECT * FROM #tmpCustomerAmount