Refund and removing installment

I bought JBL headphones decided I didn't need it. And started the return, literally the day after, this was all done. July 22nd they confirmed receipt of the return July 24th. the installment is still on my account. And I was still charged for on this bill. I've contacted t mobile three times each time. Telling me that it'll be resolved within two weeks and to wait another two weeks. And then to wait another three days, I waited another two weeks, and it's still in my account. Has anyone has this issue? How did they resolve it IF they did or how long until they removed the installment from your account!? I'm getting pissed! Ive vouched for T-MOBILE after switching from At&t and they're doing the same shit as AT&T, im considering just going to visible or mint at this point! 😤