bad endings to dog-to-dog greetings

[collies3]

What is going on!? My 3-year-old neutered male collie has a problem with ending doggie "getting aquainted" times. The problem occurs when he is on-leash and is greeting a new dog. He starts fine....sniffing politely....tail waving and friendly, but when we have to leave and I give him a twitch on the leash to signal him to come, he snaps and/or barks at the other dog. It is more than a single "yap"...he sounds and acts like he really means it.
He is fine off-leash and with dogs he knows. He understands "leave it" and "we are working" when we are on walks or at class, but I'd like him to be able to greet new dogs in situations where it is appropriate.

[LetsPlay]

Hi there,

could you please clarify this for me:
Does your dog only start reacting like this when you tug on his leash?
Has he ever reacted negatively to another dog when off leash or on a long leash?
If you meet a dog that he knows well and you tig on the leash would he do the same?

Thanks!
John

[collies3]

I see this behavior when he is asked to leave the new dog. If asked to do other things....sit, down, etc. he complies willingly, but if asked to leave, either verbally or with the leash, he snaps.
He has also tried to initiate a conflict when very, very excited, like right after completing something new at agility class, or if another dog is aggressive, but only when on leash. He has never reacted negatively off leash. Then he'll just quietly turn away or leave.
He has never reacted negatively to a dog he knows well. Again, he just turns around or leaves. If our two female collies try to take away a favored resource (the best spot on the couch, a new toy, etc.) he just stands his ground with his face turned slightly away until they leave or lets them have it if he doesn't care too much that day, and he does the same with dogs he knows well.

Thank you very much for your help with this annoyance.