Replacing an old device with a new one with an additional payment

Question:
And may Allah bless you. This sister, Umm Badr, says: I bought a mobile phone for one thousand four hundred riyals. After a period of time, I bought a new device. I asked the shop owner to buy my old device for one thousand two hundred riyals, and I bought the new device from him for one thousand eight hundred riyals. The seller said to me, “Add six hundred riyals,” and he took the new device and gave me the old one. Did I fall into something forbidden?

Answer:
There is a reversal in the question. If it means that she wanted to replace the old device with a new one, with an additional payment, with an advantage of the new over the old, then there is no harm in that, and this does not fall under riba (usury or interest). If she sold an old device for a new one and paid the difference between the two values, there is no harm in that. Yes. It would be like that if she sold and bought gold in the same way. No, not gold. Gold is what riba applies to. It is permissible to sell gold for gold only weight for weight, like for like, equal to equal, hand to hand. It is not permissible for there to be an increase in one of the two golds. Yes.