Jack would do exactly the same thing when I tried to put a harness and leash on him. I wanted to let him come out onto the balcony with us, but he refused to walk/sit/stand in the harness.
Then, when I wasn't looking he promptly backed himself out of the harness and leash (even though the manufacturer insisted that it was impossible to back out of it!).
Anyway...no "outside" for Jack!