Why pending?

If this is the first time you’ve received money via Apple Cash, you will have seven days to accept the money. You will also have to accept the terms and conditions for the service.

See the article and excerpt below for more details.

Send and receive money with Apple Cash - Apple Support

How to accept a payment

The first time that you use Apple Cash, you have seven days to accept the money that you've been sent. After you accept the terms and conditions, payments are automatically accepted and added to your Apple Cash card. You might be asked to verify your identity.

When someone sends you money, it's securely received and kept in Apple Cash. You can use the money right away1 to send to someone, spend with Apple Pay, or you can transfer the money to your bank account or eligible debit card.

If you send money to someone and they don't accept it, you can cancel the payment.

Thanks for the additional information. It’s also possible that the sender’s underlying payment method has not cleared or been verified by Apple. Try using the Get Support feature at the top of this page for further assistance.