The brevet system should not be confused with Volunteer rank.

There could be Volunteer brevets as I wrote above, but when they came with authorization to command in the brevet rank from the President or from Grant who had authority from Lincoln, they came with the pay and emoluments of the rank.

I know from the testimony of the Adj Gen in the 1870's that the President retained the authority to appoint officers to command in their brevet rank. However by that time to right to pay did not go along with it though a general officer by brevet could have ADC's.

Promotions by brevet during the Indian Wars came to a halt largely due to the animosity of John A. Logan. However in the 1890, some retro active brevets were awarded, one of them as Brig Gen going to Capt Benteen.

Brevet promotions like fukk rank pronmotions had to be approved by the Senate.