Expense AP Accrual Account default

In our company, the Expense AP Accrual Account is set - under Purchasing Options - to this particular code (e.g. 01-xxx-xxxx-xxxxxxx-xxxx); the 01 in the code designates a particular organization; when PO's are prepared for other organizations this code defaults and it cannot be changed.  Also under Purchasing Options it won't allow a null value, it forces an entry.

This is creating a situation where on a monthly basis monies in this account are moved to the correct Expense AP Accrual Accounts for other organizations.

How can I avoid the above?