Typically, new features are tested before they're released so that users don't experience changes to the app until it's stable. Sometimes, however, modifications are introduced by developers to the official version of the application, and this is also the case with the tests of the cashback function in the Google Pay application. Some Reddit users using Pixel phones report that Google accidentally, without any reason, gave them cashback in the form of cash.
In some cases, cashback is for small sums, but sometimes it is as high as 1000$. At the moment, everything indicates that the error with the accidental granting of cashback will be able to be reversed by Google, and the money will be withdrawn from the accounts. However, this will only happen if users fail to spend the money allocated to them or withdraw it from the account. In other case, Google will not require a refund of the mistaken amount.