Hi Melanie,
indeed, I don't think you are able to configure via standard Customising settings as per your requirements - a derived flow in another currency (in your particular case in the derived flow in PLN vs other transaction flows in EUR). another business question here - what exchange rate should be applicable for a specific flow?
regarding the payment in another currency - I believe this is not that complicated, as you can create outgoing payments in a required currency (e.g. in PLN) and then clear a required treasury flow in another currency (e.g. in EUR) after posting (trans. code TBB1) with this outgoing payment - it is slight modification, as usually payment happens after the posting.
talking about the first part of the issue:
- check, maybe some OSS note exist on this issue;
- if you have such an option, you can also consider to implement a User exit to handle your requirements - this requires additional resources, testing, etc.;
- the last and probably least attractive option - handle these withholding taxes via Additional flows - a few advantages here - you have full flexibility in what currency, what amount, etc. to enter, it also can be done rather simply from the implementation point of view, however posses you into a situation of potential manual errors.
hope this will help you.
Kind regards,
Renatas