We agreed that I would produce a song for this artist and that he would pay me to do so. Also I stated that I mix all my productions cause I generally don’t trust mic engineers from where I am from, I’ve had too many bad experiences. We agreed that I’d send the mix, a performance mix and a acapella mix when I receive payment, WHICH I STILL HAVEN’T.

However my fear is that now they’re requesting the SEPARATES to the song which I know they’re going to use to reproduce the song.

Well - firstly, you’ve kept your side of the agreement, so they need to pay before you discuss anything further. That’s the first thing to politely mention.

After that point - it’s kind of up to you. You’re worried they’re going to mix the track with someone else? In which case maybe open a discussion as to why.

It might transpire they actually want mix stems for remixing, or live backing tracks or something. In which case it’s up to you to either supply free of charge or charge extra, depending on the time it’ll take you and how you feel about doing it.

If they really don’t want to discuss it and just want the separate multitrack parts - you’re not obligated to provide. But again assuming you’ve been paid...again you could supply for a fee, and walk away from it.