My dog (Bernie) is a rescue and is reactive and I believe its fear based. Hes 4 years old retriever mix. I think my partner and I have done a decent job training him. But we have failed him a bit socially.

When we are walking and he sees other dogs he fixates on them and will maybe bark at them and try to get closer. Anyway, I want to talk about a particular problem moreso with humans.

We will be on leash and walking a path and say someone wants to say hi to him and pet him. Now usually I say no but sometimes ill try. he will do ok where he allows to be pet and will even listen to commands like sit and shake paw. Now me and the person will finish up our small talk or whatever. when we go to leave or disengage the becomes aggressive and even will try to bite immediately after seeming ok. So I believe hes allowing the person to get close and touch him but inside he probably super afraid and like WTF. then when we leave I most likely cause tension on his leash which I think triggers his over reaction.

How would you work on behavior like this?