This is a feature that is common on competitive point of sale systems and that was enabled on the old system that I replaced with Square.
When a credit/debit card authorization amount exceeds the available balance, most credit card issuers return the available balance if an option for partial payments is included in the transaction request.
The workflow would be so much better if Square were able to recognize this condition and offer to accept a partial payment amount up to the card available balance, and the collect the remainder with another payment form.
This is especially important for prepaid cards that essentially function as a gift card to the customer. Often the customer does not track the card balance but knows that there is some amount on the card they would like to use.
Our current practice is to have the customer use the balance lookup that is typically listed on the back of the card, but this is cumbersome and annoying to customers. And it greatly slows down processing when we are busy with other customers.
This is an important competitive feature that many point of sale vendors support.
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