Hello Sebastian,
it is not possible use the position change category "post (negative) purchase value" for the nominal flows of a cross currency swap.
Technically, the swap will have one unique position currency in position management, so it will not work to add two such position relevant flows with different currencies to that position (this is what you encounter with tpm_trl056).
FX valuation is thus not possible for cross currency swaps - as a workaround,
realized gains and losses can be created using the BAdI TPM_TRL_MANIPULATE, method MANIPULATE_DERIVED_FLOWS.
Notes 921346 and 1024279 give explanations about the customizing settings (position change cat. 1006 "indirect position change") and contain an example implementation.
Hope this helps,
best regards,
Marie