Proof

A sample of the Work used by the Holder to prove they really do have something to Ransom.

The Proof is mainly used to fight vaporware and other forms of fraud.

It can be just about anything, depending upon what the Work actually is. Here are some suggestions for Proof:

Computer software could be released as a binary-only version, with good documentation. This prevents access to the source code, and can even be programmed as "shareware" which stops working after a certain amount of time until a payment is made.
A book or research paper could be released as an overview including plenty of quotes, or simply a single chapter or section of the written work.
A painting could be released as a medium-resolution digital image with a watermark.