PBF will force packets out of a different interface than what the routing table points towards, but you cn't change the port or make a session into a proxy connection. If the Proxy is fully transparant you could connect it directly to the firewall with an ISP upink behind it and set PBF to split off all port 80 and 443 to the second ISP, pssing through the proxy while getting there.
the palo isn't a proxy o doesn't have proxy features (you can set up wccp on your switches and direct trafic as you see fit, you can set the firewall in layer2 mode with l3 vlan interfaces for routing purpoeses, this would allow you to direct traffic where you need it)
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