I'm pretty new to this but isn't that an easy problem to solve - not hand over the work until everything has been paid? Currently working on a backend project a

I'm pretty new to this but isn't that an easy problem to solve - not hand over the work until everything has been paid? Currently working on a backend project and I've asked them for an initial payment (which I got promptly) before starting the work and now I'm letting them see the project on my own infrastructure (they can interact & play around with it), but I'm not handing them over the keys until everything has been paid for.