Back from the void...
Hi, all! Hope everyone's well.
It is true that in the pilot, Sam is told he can't leap unless he completes the mission. Initially this was to give Sam a reason to play along with this notion because his FIRST goal was getting home. He could have just folded his arms and said "I'm not doing anything" and then the series would be over. Bellisario even said in early interviews that he wanted Sam to ALWAYS be successful in his missions, even if HOW he was successful was a surprise. But somewhere along the way, the writers must have realized that there's no suspense about whether or not Sam leaps--we know he has to do that anyway. But there can be suspense as to whether or not Sam succeeds. So by "Leap for Lisa," Sam and Al are talking about how they both "know" that success has nothing to do with leaping. And we're all saying, what gives? When did this happen?
Finally, one day it occurred to me. In "Vietnam," his goal was not to rescue his brother, but to ensure the success of the mission. Sam saved his brother, but the mission did NOT succeed. He did not accomplish what he was there to do, which would have meant rescuing Al. (And Maggie died too.) Sam was allowed to stay long enough to have learned his lesson and to discover that some good did come out of it (Maggie got his Pulitzer, and hey, his brother IS alive) so that he could go on with his life...but he leaped right after that. Not because the mission had been completed, but because there was nothing left to do or for Sam to learn there.
The other possible time that I can think of is "Freedom." Sam was supposed to get the father home to die on his native soul. Depending on how you interpret the scene, either 1) the father died just as they got onto the soul, thus Sam just barely completed his mission; or 2) the father died just before they got there, but Sam had done his best and the father would be buried properly at least, so Sam leaped anyway.
At least, that's my opinion after thinking way too long about this.