This shouldn't have been for the sale of anything, rare movie posters or not.  Ask your client for information about this transaction and any related contemporaneous documentation.

It's either your client was not paid for the sale of anything but that he had rendered certain services for the procurement of the collection in a non-employee capacity or that the university had mistaken the form they should issue for the sale.

It could also be this is for some other services your client had rendered in conjunction with the sale (but that would mean you are missing the amount to report for the sale).