Try with table aliases:

SELECT *

FROM [LinkedServer].[DBonLinkedServer].[dbo].[Sales Price] AS remotetable

WHERE NOT EXISTS

(SELECT localtable.*

FROM [LocalDB].[dbo].[Sales Price] AS localtable

WHERE (localtable.[Item No_] collate Latin1_General_CS_AS = remotetable.[Item No_]) AND

(localtable.[Unit of Measure Code] collate Latin1_General_CS_AS = remotetable.[Unit of Measure Code]) )