Once upon a time I used phpScheduleIt for my university’s film equiment reservation system. It has since been renamed Booked, but is still open source and available on Sourceforge. One of their features is “integrated payments and credit” but we never tested or looked into that feature since it wasn’t needed for us.
Reviving a dead thread, Snipe-IT has always had my vote too. I ran it at a solar energy company for internal IT and it worked well. The time you invest in entering meaningful data will pay off.