I got the impression from your post that you made a commitment to your new employer that you're going to start on a particular date. And your old employer wanted you to break that commitment because they had some deadlines of their own to meet.
At this point you if stay longer at your old job, you're inconveniencing two entities: (a) yourself because you're not going to look professional to your new employer if you make a commitment (or even give the impression of a commitment) to join at a particular time and then not deliver, and (b) your new employer because they may have made certain plans based on your availability.
In theory, it's possible that the work you were going to do at your old place is so important that this situation could result in a net positive even if you broke the commitment you made, but I think it's unlikely. And in any case, even if your old employer really really really wanted you around, they need to compensate the new place too, not just you!
The way I would've dealt with this would be to tell my boss something like this: "I can't stay longer because I've made a commitment to the new company and they have made plans based on my availability. However, if you think that having me around is very crucial, let's bring them into the loop as well and we can work out some terms under which you can compensate them and me for being more flexible in my leaving dates."
At this point, my guess is that a scumbag will backpedal because any stunt they pull will now be documented by multiple people.
I got the impression from your post that you made a commitment to your new employer that you're going to start on a particular date. And your old employer wanted you to break that commitment because they had some deadlines of their own to meet.
At this point you if stay longer at your old job, you're inconveniencing two entities: (a) yourself because you're not going to look professional to your new employer if you make a commitment (or even give the impression of a commitment) to join at a particular time and then not deliver, and (b) your new employer because they may have made certain plans based on your availability.
In theory, it's possible that the work you were going to do at your old place is so important that this situation could result in a net positive even if you broke the commitment you made, but I think it's unlikely. And in any case, even if your old employer really really really wanted you around, they need to compensate the new place too, not just you!
The way I would've dealt with this would be to tell my boss something like this: "I can't stay longer because I've made a commitment to the new company and they have made plans based on my availability. However, if you think that having me around is very crucial, let's bring them into the loop as well and we can work out some terms under which you can compensate them and me for being more flexible in my leaving dates."
At this point, my guess is that a scumbag will backpedal because any stunt they pull will now be documented by multiple people.