Question: In the absence of sunlight, plants give out the greenhouse gas, carbon dioxide. So why is it said that plants increase the concentration of atmospheric oxygen? Do they not increase the concentration of carbon dioxide and use the oxygen produced in photosynthesis.

Answer: 
> 
> Do they not increase the CO2 proportion?
> 
> 
> 

No, because the CO2 they emit came from the atmosphere to begin with. And the carbon they don't release as CO2 stays as carbon compounds that make up the plant. 

The issue with CO2 is not plants taking CO2 from the air and re-releasing it. That doesn't cause a steady increase in atmospheric CO2.

Taking CO2 that has been sequestered in the ground for hundreds of millions of years and adding it all to the atmosphere in a geologic eyeblink of time, that's a problem.
Plants start to grow first, from tiny size (from seed in higher plants), and from there, any size in later stages they achieve to have, they achieve it only by first absorbing all the Carbon they need, from air first.
So huge part of plant's later dried weight is actually made from carbon. And all that carbon was incorporated into it's body through continuous photosynthesis - so it was "sucked" from CO2 in the air around the growing plant.

Importantly:

Unlike us, plant can not normally absorb carbohydrates (sugars, fat etc - we eat them, plants don't) in order to use them for metabolism and respiration (ie breathing). For this they can use only carbohydrates they themselves already created first from CO2, through photosynthesis.

Even when they can not use photosynthesis anymore (because of age or in the dark) - they use for respiration only that carbon which they stored in the body before by catching it from air.

And as long as the plant continues to grow and have access to any light it continues to absorb much more CO2 than it releases through respiration. So in balance it keeps absorbing CO2 and releasing oxygen in net balance.

As an mental exercise: It would be different if You would take, let say same house pot plant and place it into a completely dark room for a long time. That plant there is without light, while it already has body which it created previously through photosynthesis, it now can not capture any light and any carbon any more, yet it would still need to be continually breathing. So as this particular plant in the dark, have no source of energy for photosynthesis, and it still is breathing, so in this particular case it would produce CO2.

Practically something resembling such theoretical example happens with significance only, when we store plant body-parts like fruits and vegetables (or potatoes) for a long time. Fruits, vegetables and potatoes (often stored in the dark) breath and not photosynthesize. 

The cycles between a day and night are also bringing plants for short time into the dark and as such they have negative balance (net producing CO2) for a short while, but it is insignificant from the whole day perspective (overal they consume much more CO2 than they produce it) 

Interestingly, there also exist very rare species of parasitic plants (they are parasites of other plants) which are not having their own photosynthesis - in this case they have no chlorophyll and suck sugars out of other plants - and yes, they only breath and yes, they only produce CO2, so they would be probably the only plants which are net producers of carbon dioxide.