Decode rs-422 without line reciever
So, your A and ~A form a differential pair – and as it's only required that the relative voltage of these reflect the data signal, a totally legal differential signal would fix A at 6 V, and make ~A alternate between 4.5 V and 7.5V (if you're going for a 1.5V differential voltage). AC coupling might also be part of the line driver - if the protocol ensures sufficient rate of state changes, e.g. through line coding, in which case the ground-referenced voltages on A or ~A might also be negative for short durations, even without anyone actually having any negative supply voltage.

So, long story short: you have differential line. Build a differential receiver!

That needn't be problematic or expensive. As said, if the protocol ensures sufficient numbers of bit transitions, just using something stupidly simple as

       +VCC
         |
       |10kΩ|
         |
A---||---+-------\   |\
         |        ---|+\
       |100Ω|        |  &gt;--MCU
         |        ---|-/
~A--||---+-------/   |/
         |
       |10kΩ|
         |
        GND


Would probably suffice to receive differentially. Whether or not you use an external comparator or a comparator built-in to your MCU depends on whether your MCU has these. (many do, but then again, if you had one, you probably would have no problem.)

But honestly: get a darn RS-422 receiver. These are 3mm × 3mm including leads and cost something between 23 and 56 ct. This seems to be a no-brainer, unless you need to cost-optimize for > 10⁴ devices, in which case the answer might be: get a MCU with a differential RS-422 receiver built in.