On the surface so many things seem wrong with your question.
- Digital pianos probably don't need to go into Mic inputs
- An AUX out from the keyboard is probably not the best way to record the keyboard
- not sure about zero resistance. Not possible I would think. Perhaps low impedance. please provide make and model of keyboard.

For now I need a cheaper way to record the sound of the piano with a computer other than buying an audio interface box. I know the AUX OUT/MIC IN solution is not the one with the best quality but it'll do for now. The piano is a Yamaha P-155.

I think the "zero resistance" phrase from p47 of the P155 manual is a rare mis-translation into English by Yamaha. They probably meant something like "low impedance" or "high-quality". On the whole, Yamaha makes an excellent job of translating their operating manuals into different languages.

I assume that the "MIC IN" connector on your computer is a 1/8" stereo jack rather than a professional XLR mono input. If you want stereo recording, you will need a lead that has a 1/8" stereo jack on one end and splits into two 1/4" mono jack plugs for the other end. Doing it this way will almost certainly overload the MIC input on your computer, but many motherboards either have a separate LINE input jack or else allow the MIC input to be attenuated to something resembling domestic line level. If neither of these is possible, you will need an external attenuator to avoid overload. Tell us more about the computer you are connecting to and we may be able to make further suggestions.

Yes, the computer MIC IN input is a 1/8" stereo jack (it's an Apple MacBook Pro). Since the MIC IN input I mentioned has an input volume control in the software system settings I am assuming I can avoid the overload you mention.

I presume from your answer that I need not to worry about the "zero resistance" issue? I was thinking of ordering the cable you mention through amazon and I wanted to make sure I ordered a quality cable.

Thanks for your help.

The audio input connector on modern MacBook Pro computers is a Line Input (+optical), and so should be happy to take the output of the P155 keyboard without an external attenuator. The Amazon cable you would need is:

3.5mm Stereo Jack Plug to 2 x 6.35mm Mono Audio Cable - 2 Metres

one, or its US equivalent, if that is where you are located. I don't think you would gain very much by spending a huge amount more on a cable.

I'm not in a position to verify, but it may be that older MBPros had true external microphone inputs, but if so, it should be possible to switch them to a line level input via the preferences.

Thank you very much for your help.