Simplify can_confirm check

_get_missing_change_value() is the same as _outstanding_value, with the
exception of ReturnedSale. So, simplify the code and use just
_get_missing_change_value that will work on all the cases.

Change-Id: If907f0e74a57877e7d0d7ced2cf247368891c821
Merge "Simplify can_confirm check"