Can we use project rev alone in proj billing?

Dear experts,

My client uses a third party application to issue project customer invoices and post the entries:

A)

Dr  Sales

Cr  Debtor

by a custom interface to oracle GL, from the third party application.  The project number, third party application Invoice number, transaction date are being recorded in journal lines DFF in GL.

Then in GL, Finance user is doing a manual 'Reversal of these Project Billing invoices', by:

Dr  Sales

Cr  Deferred Revenue

C) After a project excel work (where in user perform some calculations for current month) is done,

D) User will:

Dr Deferred Revenue

Cr Sales