I was looking at a glider that was imported to Canada from SA.
There seemed to be some expensive paperwork to deregister from SA. From the US it's free.
You may want the FAA to fax you (at a US fax number) a copy of the confirmation they send to the CAA.
At least in Canada it can take some weeks for the deregistration to make its way from the bowels of TC HQ to where the regional offices can access it.