If there's any performance planned... then Instead of smoking, and doing basically anything during her free time she should be rehearsing hardly and devoted and foccused on the performance.... otherwise it would be another "MEH" performance in her career.
So ... Giving the fact Britney seems very relaxed and lazy during her free time I don't think she is going to perform on The-xfactor.
the additional $2M would be optional, I mean she is free on choosing it or not.
Apple® today announced that BEYONCÉ has become the fastest selling album ever on the iTunes Store® worldwide with an unprecedented 828,773 albums sold in just its first three days. BEYONCÉ also broke the US first week album sales record with 617,213 sold. —Apple