This question is not exactly for us, as other hosts, or for Airbnb, it's really a question for you and the property owner you work for. You two need to come up with an agreement on how to mutually handle cancellations.
Nevertheless, some things can be pointed out. If I was the property owner in question, I would be quite upset if I discovered that my property manager was making off with a full 50% of any given booking, when the deal was that he should be getting 20%. In other words, ask yourself why you should profit and get paid more than twice your usual 20% fee, (50% = 2.5 X 20%) when a guest cancels!! Yes, it's true the property owner was not "inconvenienced" by the guest staying there, but I would think the property owner would be inconvenienced by a loss of anticipated income. Property owners go into this business because they expect certain income, they need a certain amount of bookings (which often they rely on the property manager to help get) and cancellations eat into that -- all the more so if the property manager is expecting to keep the entire payment from any cancellation!
In fact, rather than you getting to keep the whole 50% if the guest cancels, a better argument could be made for the property owner getting to keep the whole 50% and you getting paid nothing. The reservation was only made becuase the property owner had a property - HIS/HER asset is what brought in the money, not yours. Also, you as the property manager have almost no work to do if a guest cancels -- you do have the online communication part to do, but you aren't checking anyone in, you aren't checking anyone out, you aren't cleaning a place, and you aren't attending to guest needs during a stay. So what I am wondering is, since you are doing virtually no work for a cancelled reservation, why do you think you should get paid for one?
Still, this is ultimately for you and your property owner to decide between yourselves. I am just suggesting what some of his/her responses to you might be, if you asked this question of that individual.