Sum ( Payment ) will always produce identical results to just referring to Payment unless Payment is a repeating field.
To sum multiple values with this funciton you'd use either a repeating field or one of these two expressions:
Sum ( field1 ; field2 ; field3 )
Sum ( RelatedTable::Field )
Your Payment Summary field will compute the total of all records in your current found set. It thus will compute different totals when the found set changes so that may explain your incorrect results here.
To go any further than that, I'd need to know more about how this payment table is set up.