Upstream declares only host and port. Path is a part of proxy_pass. So your config should look like this:
upstream balancer {
server localhost:9090 weight=2;
server localhost:10080;
}
...
location /bal/ {
proxy_pass http://balancer/pa/;
}
Also, be careful with trailing slashes, generally you use or don't use them in location
and proxy_pass
simultaneously.