In NAV 2013R2 Sales Return

I have sale with VAT 5.5% Structure on 01.4.16 and this sale return by customer on 20.09.16 but this time VAT rate 6% .

in NAV at the time of sale return on 20.09.16 tax calculating on 6% . but i want tax calculate on 5.5%..

pls help me on this issue.