Two way matching not in match exception

Hi All,

I had a issue with matching for which i was not able to find out the Root cause . Below is the issue

The client had raised a Purchase for USD 25000/- for procuring some goods (Amount only option not checked) and quantity as 1. The matching was 2 way matching and vouchers were raised and paid to the vendors. Subsequently the user tried to increase the PO amount by USD 10000 , but the received the error " Cannot reduce PO quantity below vouchered quantity(3) on line 1, sched 1". On subsequent analysis it was revealed that vouchers to the extent of USD 35000(5 vouchers) have been already paid against the said PO and all the vouchers had been successfully matched. This