Although billing reverse is done, sales order pricing is inactive

Hi,

I have a problem about changing price on sales order.

I have created a return sales order (without reference), PGR and billed. Before creating accounting document, I get reversal of invoice and try to change price in sales order but conditions tab is closed and can't change prices.
I can't cancel PGR but it already didn't relate to it.

Thanks,

Oktay