We do this with clients where we have particularly strong relationships. This is particularly helpful when clients are getting the budgetary pinch or if they've run out of money before their budget periods are over.

There is a definite risk that comes with this which is that a client you're not familiar with who's short on cash may take advantage of this, so I'm not convinced it's well suited to everyone's needs but where there's mutual respect and trust this type of pricing model (and flexibility) can really cement bonds with customers.