Voucher not eligible for Deletion error - open amounts not equal to gross

I have a posted AP voucher which needs to be voided but I get the 'Voucher not eligible for Deletion' error. 

The voucher had been paid and there has been an error with the payment void.  There are no payment records relating to this voucher in the F0413 and F0414.

There are 2 payitem lines on the voucher both 001.  One is a PE adjustment.   For both lines the open amount does not agree to the gross amount.

Gross amount       Open amount

2,610.75               -2,610,75

5,221.50                nil

What is the best way to correct the open amount field (AAP) so that the voucher can be voided?