You will be able to see your purchases in both apps: Google Pay and the MuchBetter app.
Please note that the transactions in Google Pay will always display the original amount in the local currency, whereas the MuchBetter app will display it in your wallet currency. If you wish to see the transaction details, just tap on the payment in question in Google Pay. You can do the same in the MuchBetter app.
Hint: If you have more than one card linked to your Google Pay account, you will still see all charges in your Google Pay account made with all cards. You need to click on the transaction in question to see what card has been used for this purchase.