Kamma cannot be undone — but its results can be lightened. The Pali Canon says how
Summary
When life is blocked, when misfortune arrives, when it feels as though old karma has caught up, many people believe they must go and have it cut, transferred, or removed by some ceremony.
The Buddha taught none of that. Not cutting karma by ritual, not transferring it, not shortcuts by which old deeds disappear. What he taught is deeper: the deed already done may be beyond erasing, but its result can be lightened — by developing oneself in body, virtue, mind and wisdom.
The key text is the Loṇaphala Sutta and its image of salt and water, which explains why the same deed does not produce the same result in different people. If the mind is a small cup, a pinch of salt makes it undrinkable. If the mind is as large as the Ganges, the same lump of salt cannot change the taste of the river.
Key points
No transferring of karma and no shortcuts
What the Buddha did not teach matters as much as what he did, and here he taught none of it.
The deed cannot be erased; the result can be lightened
The difference between those two is the whole point, and it is what makes practice meaningful.
The same salt, a different volume of water
The Loṇaphala Sutta locates the variable in the vessel, not the salt.
Enlarge the mind through body, virtue, mind and wisdom
These four are how a cup becomes a river — and all of it is work one can do oneself.