How to avoid unauthorized charges on Cash App in App Store

One possibility is that there were some outstanding charges unrelated to the app he downloaded. This is primarily a user-to-user support forum; other Apple users like you. We don't have access to information about charges to your account. Apple has a support article about charges — see each of the steps in the following instructions. They include information on contacting Apple Support if the other steps don't help. See: If you don't recognize a charge - Apple Support

karman131 wrote:

my dad downloaded cashapp and Apple asked for a payment method to download the app (cashapp is free) and then proceed to cut $11 two times on his card. (bold mine)

It's possible he had an outstanding balance as FoxFifth wrote. If you meant to put '$1' instead of '$11', however, then that's normal. Apple can require a valid payment method (which cannot be a gift card) for verification purposes. It may be charged a small amount to verify it but this will be automatically refunded. If it was indeed $1, it's likely the verification charges and Apple won't actually take the money.