I don't know if this is a result of a patch or old news, but it seems if you are on a mission and drop into a USS and get one of those Alternative Offers, if you DO NOT open you mission/Transactions Tab whilst in the USS the original mission remains unchanged once you jump out/enter Super cruise.
No risk of abandoning the wrong thing, in fact it seems that by opening the Transactions tab when they make the offer it is tacit agreement to their offer, thus ignoring them is the way to decline.
I have run in to issues previously on free slave missions where once I look at the alternative offer I could only follow that mission branch and not the original, Killing the person who makes the offer didn't seem to make a difference, but not this way seems to work perfectly.
No risk of abandoning the wrong thing, in fact it seems that by opening the Transactions tab when they make the offer it is tacit agreement to their offer, thus ignoring them is the way to decline.
I have run in to issues previously on free slave missions where once I look at the alternative offer I could only follow that mission branch and not the original, Killing the person who makes the offer didn't seem to make a difference, but not this way seems to work perfectly.