When exporting something with significant commercial value that will be returned to the U.S., it may be possible to arrange for it to be treated as a temporary import by the destination country and avoid having to pay import duty and VAT, if applicable.
Many countries will accept a carnet (effectively a merchandise passport), which facilitates duty-free entry into the destination country, for temporary imports of up to a year. It's important to export the item on time and to make sure the export transaction also uses the same carnet to close out the temporary import — otherwise you could be charged for duty and VAT, and these problems can be difficult or impossible to rectify after the fact.
The alternatives are more cumbersome. It is possible to apply for Temporary Importation Under Bond (TIB), in the destination country, or you can pay the duty and VAT on import into the destination country and apply for duty drawback (refund) after the item is returned to the U.S. Either of these should be arranged with a customs broker in the destination country.
Contact the MIT Export Control Compliance Team at firstname.lastname@example.org, or by phone at 617-253-2762 (Janet Johnston).