Is an open collector is for the output of a chip? Sorry but I'm not that experienced, I dont really follow
Usually called Open Drain and would be available as an I/O pin configuration setting.
Honestly, though, I don't think you're powering your whole main module through a GPIO pin. A bit can go through it, sure, but depending on your main module, that could easily be tens or a hundred mA continuous, which I don't think your GPIOs are going to supply readily. I'd check your other connections and verify that there isn't some extra power connection going through your wiring, and if not that, make sure there isn't anything in the bus protocol connecting the module with your powered one that would force the main module to wake up when talked to.
Very difficult to say without schematics/pinouts of what's being used, I just have my doubts that a substantial amount of current is powering your main module through the GPIO without damaging something on the sending or the receiving end.