What do you mean released back into the pool?
You can either 1:1 NAT inside to outside address or create a pool of outside addresses and let algorithms determine which outside address to use for outbound requests.
You can, however, tell outbound NAT to use the same outside address for connections from a particular host until there are no states left from that host.
Round Robin/Random with Sticky Address:
Selects an address at random, but maintains the same translation address for a given source address as long as states from the source host exist.
This explains it all:
https://portal.pfsense.org/docs/book/nat/outbound-nat.html
Also:
https://doc.pfsense.org/index.php/Outbound_NAT#Address_Pool_Options
Lots more options there. One of which might be a better fit since I can't really tell what you're asking.