Considering that the card has memory on it, you can store there how much balance you have when you do an online payment. The bank can send back your available balance, so you cannot spend offline more than you have.
I can't think about anything simpler than this.
I remember writing an app in Java to read the balance on a card with my laptop which had a built-in smartcard reader, because I was too lazy to go to a station. Everyone in the classroom then promptly asked me to check their balance... and a few asked if I could top it up somehow.
My thinking is that in this day and and age, unless something bad really happens (war, volcanic erruption), the chances of using a card for offline transactions for an extended period of time are very close to zero.