Credit and Domain Charging

Jaromir, We are having a small problem,I Tanzania we have sld's zones for .tz i.e. co,or,go,mil,ne,sc. Now the problem is if we add credit we must specify the zone we are adding credit.So if we register domain and the credit in say co is finished while another sld say go has credit the domain registration will fail since no enough credit in the respective zone. Now I am thinking of customizing such that if a registrar has credit in the other sld's the credit can be used to register domain in another sld. Please advice if it's possible.And if its possible let me know how can I start to do that.Which modules etc will I have to look at to get the Insight of what happens when the domain is being registered. Thanks in advance. Regards, Bryton