Get single records when duplicate records exist in 2 table
You can try to find highest sale for each email and then join it with leads:

WITH maxsales AS
(SELECT 
sales.*,
ROW_NUMBER() (PARTITION BY sales.email ORDER BY Sales.Sales DESC) AS rn
FROM [dbo].[Sales]
) 
SELECT maxsales.email as [sales_school]
  ,leads.email[leadds_insti]
  ,maxsales .*
  ,leads.*
FROM maxsales 
INNER JOIN leads ON leads.email= maxsales.email
WHERE maxsales.rn = 1