cancel prepayment line tax

hello

i need your help o solve this problem

i have created a prepayment invoice , but unfortunately a line of tax applied this this invoice and i have made the amount of 0

now at this time i need to apply the prepayment to standard invoice , but it calculates also tax and add tax line , and it's not available to cancel this tax line or modify amount to zero

if any one can help

reply to me