Osoro,
As fiscal (and political), decentralization is historically, and invariably, a result of political negotiation, the situation that you have so accurately identified, is inevitable. Among the best real-world solutions I know of are agreements built into the Decentralization Act to revisit in specific, limited parameters, the results of the initial (or current) design after specific time periods. This is rare - most reviews are a result of political pressures. Ghana, for example, was an early example of decentralization and was regarded as a model when it was initiated. However, the evolution of the process was frozen for decades before it was resumed in the last decade. Given realities, I suspect that the best approach is to recognize the necessity and realism of this evolutionary process and design a system that has strong core principles, and performance indicators that are used as the basis for periodic, infrequent refinements. I do not know of a model that was carefully planned and executed on that basis. Technically, I have little doubt that it is doable; politically? Not as confident...
Fitz.