Ask HN: Self-hosted alternatives to Concur?

I'm tasked with setting up a platform for our clients that allows them to book travel to our warehouse, and we pay for it (up to a certain amount). Rather than have the clients pay and we reimburse them, we'd like to pay the travel vendors directly. What off-the-shelf system can handle something like this? I only know of Concur that we used at our last company internally, but it seems a bit enterprise-y/overkill for our use case. If there isn't an off-the-shelf solution, any recommendations on what components to use to build it?