SELECT ISNUMERIC('-1') -- the result is 1
SELECT ISNUMERIC(',-1') -- the result is 0, this shows that the comma is not ignored

SELECT ISNUMERIC('') -- the result is 0
SELECT ISNUMERIC(',') -- the result is 1; if ISNUMERIC() ignored the comma,
                      -- the result would be 0

if( select isnumeric( @PrincipalAmount )) = 1 
	select convert( float, @PrincipalAmount )
else
	print 'Sorry'
;