Air carrier Captain reported the aircraft rolled toward the tug after brake release during pushback. Captain applied brakes to stop and then tug captured the nose wheel.

Narrative

While we were waiting for the ground crew to push the aircraft back; we realized that the crew's headset didn't work. They initially gave me the brake release hand signal (which I initially declined with a thumbs down). The ramp agent then went to find another headset. The ramp agent then came back and asked for brake release again; which this time with the weather being day; VMC; clear; and no clutter with both the FO and myself being okay with it; released the brakes for them. I returned the brake released signal; and at this point; the ramp agent completely began to walk away from the aircraft out of sight. I started to see the aircraft begin to move forward ever so slowly. I couldn't see a person in the tug through the reflection or window; so I had a feeling that maybe we were rolling towards the tug. I was uncomfortable since the ramper had left and we couldn't see anyone outside. I wasn't sure if someone was in the tug; so I slowly and softly applied brakes via the pedals. Right as I did; I felt the nose wheel raise (being captured by the tug) that I released the brakes as I knew someone was in fact in control. No abnormal indications occurred and was verbally verified with the crew once headset communication was established. The pushback was uneventful afterwards.