Would this sell/trade mechanic break the rules?

I have an idea for a tycoon-like mechanic where you can sell a resource for coins/gold, but to make it more interesting and with an added depth I wanted to set the conversion rate to a random number between two parameters. For example, you can sell 1 resource for 20 coins, but 10 minutes later it changes so you’d now get 18 coins for the same resource, 10 minutes later it’s 22 coins etc.

I was wondering if this could be flagged for gambling and rejected. Obviously that’s not the intention I just didn’t want to make a flat rate that stays the same all the time, as there’s then no benefit to sometimes saving the resources up and waiting for a better time to trade it in, but I understand how it could be seen as a grey area and I want to make sure I’m not breaking any rules.

Does anyone know if it would be allowed or not?