The frontman, who bagged 21 goals last season, has been subject to rumoured bids from a host of clubs since January, with Millers boss Steve Evans old side lodging numerous enquiries.
The hottest interest came before the Millers final home game with Northampton Town, with Evans saying the situation with his top scorer would be discussed at the end of the campaign.
It now appears that Grabbans future has been finalised and with the Millers agreeing a fee with the Sussex club, the player is in the process of agreeing personal terms.
More to follow as we have it.