How to reverse the effect of what is charged to Customer

Hi,

We have a scenario:

- One sales order created with two lines ;

- First line is inventory item, shippable

- Second line is service item non shippable

Second line will get process when line one is processed.

User cancels line one first then he tried to cancel line 2; system errors that line has been closed.

Line two get processed and charged to customer; which shouldn't be.

This line processed in AR also.

How we can reverse the impact of this line on user account.

Regards.