http://www.clarityaccounting.comhttp://www.blinksale.com
http://www.freshbooks.com
Now, more importantly:
The startup could sell the software to the company on the basis that they don't have to pay for it until the company can afford it.
Dude WTF? Not trying to be harsh, but this is known as guaranteeing the failure of your business. Most businesses fail. That means most businesses will never be able to "afford it". Ergo, you will never realize a vast majority of your potential revenue.
Further, your suppliers won't be as charitable as you are with your clients. You'll be paying bills to support clients that aren't giving you any money. You won't have income. You'll have increasing expenses. You'll have a disincentive to actually trying to growing your business. Disaster all around.
The goal of business is to make more money than you expend. If you want to be nice, found a charity - even then you'll need to brake even one way or another.
An overall better option would be to do as my linked examples have, and provide less of a product for less of a price. Most startups simply have no need for Accenture style services hence, they can purchase something that better fits their needs.
Do yourself a favour though and drop this "pay me when you can" idea forever. You'll have more than enough risk in managing your own profit levels, don't take responsibility for everyone else's too.
I know I'm being very direct here, but this aversion to making money is something a lot of people have, and it kills businesses every day. It's important for not just you but all of us to avoid this position whenever possible.