sps-530 need new firmware

I have a sps-530f.

It has mix & match function.

However, I find a bug.

For example, if sell 1 apple in 3$ and 2 apples in 5$, there is a discount $1 when buy two.

When entry 2 items together, it show discount and give the right amount 2*3$ - 1$ = 5$

When entry one by one, it shows discount but does not give the right amount, 

It still show 3$ + 3$ - 1$ = 6$. it just ignores the discount. 

Even press the subtotal key, the total amount still 6$.

I wonder if there is any newer firmware for this machine.

Any help will be appreciated.

Thanks.