Comment by vertex-four

You can find multiple buyers before you write the software, you know - you don't have to have just one person paying you. If it's consumer software, things like Kickstarter or Patreon are how this is often done - if it's b2b, this is how a lot of software development is done anyway. Examples of software successfully funded via crowdfunding include Spine, Magit, and Diaspora - although Spine appears to be closed-source anyway. You can write an MVP, pitch it to potential clients, and write software that provides value to them. That's what I'm doing.