When used Tax rates and when used BI Middle rates?

For Example there is a sale on January 1, 2011 with a value of $ 1,000, BI Middle rate is Rp 8,500, and Tax rate Rp 8,700 

Account Receivable (Dr.)                                9,370,000
     Sales (Cr.)                                                8,500,000     (1000*8500)
     VAT-out (Cr.)                                               870,000     (100*8700)

Total bills / receivable is USD 1,000 (Recorded amounting to Rp 8,500,000) + Rp 870,000. VAT-out already converted to Rp using tax rate (1 USD = Rp 8,700) So that, Rp amount for VAT-out not change although the receivable had paid. Then when the account receivable has been paid  by the customer, the account receivable will credit USD 1,000 and recorded use BI middle rate when the cash received. The difference recorded as Foreign exchange expenses (Income). For example, account receivable paid  on January 15, 2011 with BI Middle rate 1 USD = Rp 8,800, so the illustration as below :

Paid on 15-1-11 (1 USD = Rp 8,800) :

Item price              = USD 1,000 * Rp 8,800 = Rp 8,800,000 (rate when the receivable realize)

VAT-out                = Rp. 870.000

recorded amount   = Rp 8,800,000 + Rp 870,000 = Rp 9,670,000

and the journal entries as below :

Cash / Bank (Dr.)                                             9,670,000

     Account Receivable (Cr.)                             9,370,000

     Foreign Exchange Income(Cr.)                       300,000  

Conclusion :

Entry normal transaction uses BI Middle rate, while transaction related with taxation such as VAT, income tax, Withholding tax (Art 21, 22, 23, 26) uses Tax rate which are predetermined by Minister of Finance.

As above explanation, VAT-out amounting to USD 100 used tax rate Rp 8,700, so tax which will pay fixed amounting to Rp 8,700,000 (USD 100 * Rp 8,700) even though rate when the receivable realize is Rp 8,800