My novel is currently only for sale in Canada. I recently received an offer from a US publisher. Can I use the same cover with a new publisher?
That will depend on the contract your Canadian publisher has with the cover artist. If they have the right to that image for the Canadian edition only, then your US publisher can buy it from the artist for the US edition. If the Canadian publisher owns the international rights, then the US publisher can buy it from the Canadian publisher. I would just ask the artist or the Canadian publisher. They will know what their contract says. (Also make sure the Canadian publisher doesn't own international or US rights to your story. But I guess you already did that.)
And congratulations for your success!