How to Shrink Memory baloon in guest ?

Hi.
About balloonin.
Guest had min 2GB and max 15GB of ram.
Over high usage it grows ram up to 15GB of max size. And it it working ok.
But now i want so shrink this balloon back, how can i do this ?
i am dooing
echo 1 > /proc/sys/vm/drop_caches
echo 2 > /proc/sys/vm/drop_caches
echo 3 > /proc/sys/vm/drop_caches

in gues machine. and the real usage of memory is 4GB / 15 GB

How can i shrink balloon so that Proxmox will only use 4GB and free up the rest 11GB ?