We have 2 POs which have not been fully invoiced but a calculation on the invoice workform is wrong

So 30014467  Value Invoice 12398 for 20,323.55　

But the invoice workform shows a total billed of 31,265.24　

In iProc when you query the requisition the correct amount shows (in this case inclusive of VAT), 24,388.26　

next 30014484  Value   Invoice 12367  for 17,295.42  and Invoice 12397 for 20,323.55. Total invoiced 37,618.97 (all these exclude vat)　

But the invoice workform shows a total billed 64,879.40 　

In iProc when you query the requisition the correct amount shows (in this case inclusive of VAT), 45142.76　

I have seen examples of this before but the invoice had not been paid so AP could cancel and re-enter the invoice which solved the problem. In these two cases the earlier invoices are paid so cannot be cancelled. Is there anyway we can change the total billed amount as this is incorrect and preventing the remaining invoices from being processed.