[porlw]

One problem is preventing the client from making use of the software when there is a dispute about payment.

Due to the laws regarding unauthorized access to computer systems, it may not be legal for you to remove or even communicate with your software once it has been deployed to the client. For internal systems it might not even be physically possible.

To combat this, I like to incorporate a "dead man's switch". This means that the software will stop working within a certain amount of time (say 30 days), unless I actively intervene. The switch is only disabled once final payment has been received.

A notice somewhere on the site or in the app will say "Evaluation software - nn days remaining", so the client is aware and gets a countdown.

The switch doesn't have to be super sophisticated but it should incorporate some element of encryption - that way if the client attempts to circumvent it you can threaten them with a DMCA claim.