That works.
Can you "top up" a cashu token, or does it need to be an api token?
Here, paste the same cashu token here and it'll show you your balance and a button to refund.
I recommend using the API Keys section in the chat app here: https://chat.routstr.com to store (stored using nip44) and manage API keys.
Discussion
Yes. Your cashu token is your API key. They are interchangeable.
If you started a session with a cashu token, that cashu token becomes your primary API key. Even if you top-up the balance of this original cashu token, it'll still remain your API key with balance, the cashu token you used to top up will be spent and useless.
You can of course get a smaller API key using your original cashu token as well. (like sk-asdfasd)