Hello Alexander,
My understanding of your questions:
you have set percentage (%) as the method for Discount price component in the price strategy, now you want to enable both percantage and fixed amount for discount, so you can choose to either maintain the % or an absolute amount as discount in sales documents.
If it is the case, then I'm afraid that it is not supported in the current releases. It is not possible to mixed both options (fixed amount and percentage) in sales documents, only one selection is supported at a time. And note, once the price component (discount) is used, it cannot be changed anymore.
Kindly check the case document (1871107) for more details.
Thanks,
Adie