Air carrier Captain reported a momentary brake release prior to sending the DG acknowledgement form. The brakes were reengaged due to late arriving bags in which the acknowledgement form was sent.

Narrative

I didn't send ACARS Dangerous Goods Acknowledgement Code prior to releasing the parking brake per FOM 90.20.2.After a gate change and a tail swap my FO and I preflighted Aircraft X for our flight to ZZZ. Upon arriving on the flight deck; I noticed the planned DG (Dangerous Goods) summary; planned PNF; final summary; and final PNF had already printed. I took note of the DGs and retained the paperwork in the same location I store the Pilot reconciliation report. Shortly after beginning our pre-flight; we became saturated with clearance issues from the tail swap; a second reroute; and several releases finalizing in release 6. Mentally I checked my DG accepted box when considering the bulletin requiring retention of both a final summary and final PNF. I released the parking brake and we received an ACARS message to not move the aircraft until sending dangerous goods acknowledgement code; which was fortunate because ramps also informed us of a late bag arrival. We cancelled the push; set the parking brake; sent dangerous goods acknowledgement code; loaded the bag; got new final weights and reran the before push checklist to ensure we did not miss anything else.