PO line not completly cancelled

We need to completly cancel PO lines

Here is the situation:

A prepayment was done on a PO. It was for 30% of each line.

The buyer then cancelled all the lines.

Doing this only cancelled 70% on each line.

The cancelation was supposed to be done after we cancelled the pre payment

Now the pre payment is cancelled. but we are unable to cancel the 30% remainig on each line.

How can we cancel the remaining quantity/amount on those lines?

Thanks