Correction: He will forgive the sins of us who believe on His Son, not because we have been absolved by a priest or performed extra duties.
I did not say otherwise, though Christian groups differ on this issue.
Also, mere belief is not enough. The Bible says the demons believe and shudder.
Leviticus 27 is about what an Israelite must do if he changes his mind or is unable to fulfill a vow. Which is pretty much the issue here. Apparently the Law made it pretty much a financial issue.