How to resolve this Adjustment scenario?

Hi Gurus,

there is a scenario that, the original invoice has two lines,

Line one : $100 with tax
Line two: $2000 with tax free

User made a adjustment in invoice level to adjust to zero, now if they want to reverse the adjustment, how to do it?

I know we can only reverse the adjustment with the line level, however, how to deal with the tax?

Thanks,

Faye