Can you cut into post tension slab?

Of course you can. But you risk cutting into a post-tension cable. That would be very bad, but there is no reason why you cannot physically do it. Just make sure you have another job lined up beforehand, and that you are prepared to live with the consequences if the cable flies out the end of the building and cuts somebody's grandma in half.

Or, you can have an imaging company use X-ray or radar to locate and identify sub-surface items. Then you will know what is in the deck you are cutting. Here's one: www.srimaging.com