Yeah, sounds like a bag of holding or portable hole would do the trick, though a bag of devouring would do it even better.

Strahd is a genius though, and may very well understand that there is bond between the Knight and the weapon, and as part of the trade may require a ceremony of the Knight that ensures that bond is broken before agreeing.

ncG1vNJzZmivp6x7prrWqKmlnF6kv6h706GpnpmUqHy4u9Slm2aonJawqrrGZphmr5WWvbC6jKKlZpldl66oec6fZKGnnJm2r7OMm6memZtir627wqRknqSUp7a1r8dmoqehl53BtHnBqKWdZmZuf3OFlmg%3D