Why not have a "wallet" private key address on the Private side of the ring?
I don't keep all of my money in my physical "wallet", I wouldn't do it either with my ring.
So, I move coins from my "bank" to my "wallet". My "wallet" private key is stored offline, in my ring. I go out and shopping and if I want to accept a payment, I provide (or they scan) the public side of my ring and they pay me, it goes into my "wallet" just like it would with my physical wallet.
If I want to pay someone, it's a two-step process. My mobile device reads the Private side of my ring, and scans the private key. It then uses that private key with an Application that makes the payment (possibly scanning the payee's ring to get their public key), and then deletes the Private key from the mobile device after payment.
If I don't have enough funds in my "wallet", then it's no different than if I didn't have enough funds in my physical wallet, I cannot buy it.