According to the Bhagavad Gita, I am a soul; not my body. Souls are parts and parcels of God (or for advaitins, souls are God).Souls transmigrate from one body to another(2:13), but the material body perishes. Who then is my ancestor referred to in chapter 1, verse 41? Also, it is implicit that one soul can help another after the latter has left its body (by performing certain rituals; 1:41). Can a soul help another with its karmic burden?
God is believed to be the recipient of all fruits of all action. Your ancestors have given and and received karma phalams through a "divine Corpus Fund" by such routing but individual "trading" does not always succeed.
Some great souls who attained spiritual liberation have the capacity to take the energy/phalam on themselves and re-distribute to the deserving but Karma phalam cannot be transferred either as reward or punishment for and by ordinary souls through individually maintained "savings accounts"... it is into the Corpus Fund that all action credit goes in and gets redistributed in a mysterious way.
This message runs throughout Chapter 5 of Bhagavad Gita. Verse 8-9 in particular enjoins anyone with such communion as one already liberated, blessed with a joy to go further on with an assurance that whatever happens is quite acceptable.