If one eats e.g. a 300 calorie salmon patty, then eats a 125 cal.
plum, how many calories of the patty and plum itself will be 'lost' in
the bathroom e.g. an hour later, vs. how many cals. would be 'kept' of
the patty if the plum were not eaten?   Actually, I'd really like a
'general' answer in that I want to know whether calories e.g. in
fibrous foods (whole wheat bun, small salad, etc.) cancel out
absorption of what percentage of other food eaten directly
beforehand...  I don't need an exact cal. count of the plum or patty,
just the general principle.  Hope this is clear!  I don't want to fool
myself re eating too much and hoping it will 'go away' if I follow up
with fast acting fiber!

Eating fibrous foods does not 'cancel' anything. It may cause your
bowels to move more smoothly, but that doesn't mean that calories are
not being absorbed. Fibrous material, being indigestible, does not
contribute to the calorie count of a food, but neither does it reduce
the number of calories in the non-fiber portion of the food.

I don't know exactly what would happen in that case, but I think you
probably want this info:

I assume you are asking this because of "no/low-carb" diets that tell
you to subtract fiber from total carbohydrates to get "net carbs" for
dieting purposes.

Fiber IS a carbohydrate, which you can see by examining a nutrition label:
http://www.cfsan.fda.gov/~dms/foodlab.html
...note how fiber is indented below carbohydrates, indicating it was
used to get the total above it.

Here are descriptions of a whole bunch of different carbohydrates,
including the polysaccharides known as fiber:
http://www.sparknotes.com/nutrition/carbohydrates/section1.html
(OK, you can skip that big long page :) )

Not all carbs make you fat.  Some (fiber types) serve completely
different purposes.

Therefore, the fiber listed on a label doesn't cancel out anything
else, you only subtract it because the total carbs listed was not the
number you wanted to begin with.  Adding additional fiber would cause
the total carb count of your meal to go up, then you could subtract it
again.  It would not excuse the sugars already in there.  A plum also
has non-fiber carbs, so total weight-gaining carbs would also go up.

(A single plum does not actually have a whole lot of fiber...
http://www.whfoods.com/genpage.php?tname=foodspice&dbid=35
...but let's pretend it did, because plums aren't really what we care
about right now.  Let's pretend a plum had a good 33% RDA of fiber
instead of 5%.)

So eating a plum on top of a full meal will not reduce how fat you
get.  In fact, since eating only plums/fruits does not cause people to
starve to death, adding a plum would make you even fatter.

However, there are other benefits to eating fiber besides it not
making you fat, so you want to eat a plum, you just want it to be
included in the meal you were already planning on eating (at the
desired calorie level), especially since a piece of meat had none at
all--just a lot of protien and fat:
http://www.bigy.com/content/seaf/nutr/alaskasf_salmonnutr.php

If you want to diet and lose weight (I assume you do because of your
question), the first thing to look at is the calorie count.  It will
not go away.  Calories are not things.  Calories are a measure of
things.  The calorie count tells you how much energy your body will
get from whatever is in the food, and anything that your body can use
for energy can be turned into fat if not used.

Counting calories is "the only diet plan endorsed by the First Law of
Thermodynamics"
http://bobpark.physics.umd.edu/WN04/wn081304.html

http://www.caloriesperhour.com/forums/forum19/75.html