Vadim,
May be, being a programmer I'm unable to provide sufficient information in the requested way 
.
To me, it's just transfer of funds from one bucket to one or more different different fund bucket/s that refers to a particular document. Once a transfer is initiated, the document status changes from DRAFT to POSTED. End of story w.r.t my scope.
Would you be able to ask specific questions for which I may try to get answers?
From whatever communication we had so far, is it better to get the master data change done using custom logic badi instead of the write back badi given these two are the only options.
If possible, could you point me to some scn thread wherein similar questions/answers about business requirements came up and scenarios were explained clearly. If so, I guess I could begin thinking from a business process point of view rather than my technical biased thought process.
Thank you in advance,
Naveen