That is a very unusual situation, in my experience.
I've both provided and purchased annual maintenance contracts for software, and have never heard of an "early termination" option in a one - let alone one that includes a refund of monies already paid. Would McDonald's give you a partial refund on a half-eaten hamburger just because you decided you were full?
Also, remember: you're a software outfit, not a bank. Here are a couple of ideas about this with regard to taking monthly payments, rather than a lump sum up-front:
You're effectively lending them money. So you might consider saying, "If you go for monthly payments, the annual cost will be X% higher than if you paid up front."
You need to be compensated if they don't make their payments on time. An example: payment terms on monthly invoices are net 5 with interest on overdue invoices accruing at 2% per thirty days or fraction thereof, and if an invoice goes overdue sixty days, the entire contract accelerates into a balloon payment.
That may all seem rather harsh, but it's actually pretty standard.
An option you might offer would be a renewable three-month maintenance contract, with each three months paid up front. In that case, the amount you get per month should be higher than if they commit to an entire year.
That's how my martial arts dojo does things. You can pay month-to-month, or you can pay 3, 6, or 12 months in a lump sum, with increasing discounts. It works pretty well for everybody.