Perform Aggregate in Where clause
Just like the error says, you can't have aggregates in the WHERE clause.  WHERE is logically before GROUP BY, so it filters rows before they are grouped, not after.

For filtering the groups, use HAVING (or push the GROUP BY into a subquery).  eg

Declare @Hamburger Table (empName varchar(200), e float, a float, saleid varchar(10))

Insert Into @Hamburger (empName, e,a, saleid) Values
    ('Jake', 362, 233, 'E111'), ('Jake', 1431, 2702, 'E112'), ('Blue', 849, 280, 'R222'), ('Blue', 1418, 299, 'R390')

Select empName, e, a, saleid, AmtGained = Sum(Coalesce(e,0))-Sum(Coalesce(a,0)),
DiscountPercent = Sum(Coalesce(e,0)-Coalesce(a,0))/NullIf(Sum(Coalesce(e,0)),0)
From @Hamburger    
Group By empName, saleid, e, a
having ((Sum(Coalesce(e,0)-Coalesce(a,0))/NullIf(Sum(Coalesce(e,0)),0) &gt;= .70)
OR (Sum(Coalesce(e,0)-Coalesce(a,0))/NullIf(Sum(Coalesce(e,0)),0) &lt;= -.70))
Order By empName ASC