how do you cancel a pending purchase in iTunes

I have a pending purchase from the app store. I have not downloaded the purchase and I would like to now cancel the purchase.

Technically, the purchase is not pending, as soon as the payment gets cleared from your bank account the purchase is deemed complete and final. Nevertheless, you can always contact iTunes support through Report a Problem and get refund for your purchase. Some developers bar refunds if the app has been downloaded at least once, since you haven't done so you need not worry about this caveat.

The portal shows purchases you made in the last 90 days, for older purchases you'll need to contact iTunes Support directly.

Follow these steps.

1. Go to Apple's Report a Problem portal and sign in using the Apple ID used for purchase.

2. You should see a list of your recent purchases (last 90 days). Find the app in question and tap on report.

3. Describe your problem and click submit.

Apple Support will write to you with 3-5 business days. All the best.

Reference: Apple Support

Call Apple Support once it goes through and ask to speak to iTunes/billing, they can refund it as accidental. Or you can call the bank to decline that transaction all together.