I don't think you understand what a buffer is. A buffer is basically a chunk of memory where you can put data while it's waiting to be added to the OBS frame.
Think of it like this: let's say you and your friend are peeling and slicing potatoes. You peel, they slice. You are peeling away, and you put all the peeled potatoes into a bowl. Your friend grabs peeled potatoes from the bowl to slice them. The bowl is a buffer: it's a place for potatoes to wait until they are taken by someone else.
In the same way, the webcam buffer is a place for frames from the webcam to go before OBS grabs them to add to each frame that gets streamed out. If you have a bigger buffer, then OBS will wait for the buffer to fill before taking frames off, starting with the oldest one. So by increasing the buffer, you increase the amount of time before OBS starts grabbing frames, which is what causes a delay. OBS uses a buffer for some devices out of necessity due to the way the devices work, and the ability for a buffer to delay a webcam is honestly kind of coincidental.
Thus, it doesn't make sense to ask for a "negative buffer" -- that would be like asking for a potato bowl with negative space for potatoes. You would have to do what Jack0r said: delay everything else. And that isn't really very practical.
