Change PO delivery costs after reversal of GR

Dear colleagues,

Delivery costs are manually entered in PO. After PO, we are doing GR. Often it happened that delivery costs are entered with mistake and need to be corrected. We cancelled GR and after that tried to change delivery costs in PO, but it is not possible (gray field) and it is not possible to add new delivery cost even all GRs are reversed. Condition type’s condition category is B Delivery costs, and we need it because vendor for delivery costs is different from vendor for the goods.

For the business is only acceptable to change condition value in PO, but not to delete PO item and create new one or to adjust price via invoice.

Could you help me how to handle this situation?

Thank you,

Harry