How do I free memory obtained by sbrk()?
You need to use brk or sbrk again to shrink.
In the end the only way you have to modify the amount of memory(apart from mmap like syscalls), is to increase or decrease the heap, so you move it up with sbrk or brk and you move it down with brk or sbrk with a negative increment.
Don't use brk and sbrk. It's pretty much impossible to know which library functions might call malloc, and could change over time, so even if your program works now, it might break when somebody upgrades libc. They were excluded from POSIX for a very good reason.