A minor payment dispute could then hold up an investment round or M&A transaction, and that's completely out of proportion to the issue you're trying to address

A minor payment dispute could then hold up an investment round or M&A transaction, and that's completely out of proportion to the issue you're trying to address. If it's minor payment dispute, they they can just pay the invoice and get on with the M+A transaction. Personally, I can't see why the client should expect to own work done at their request if they haven't paid.