Visa X2, want to travel to HongKong, and come back to Beijing for a short day

Hi!
I'm currently living in Qingdao. I'm living China on the 26th from Beijing to go back to France. Before leaving, i'd like to visit Hongkong. As a french native, I know that we don't need any entry permit. But once i leave, my X2 won't work anymore. I planned to come back on the 25th of august from Hongkong to beijing in the night and to a transition in beijing airport to take my other flight to go back to france.
How can I deal with it? I heard about 72 hours free transit.
Thanks