Disgruntled activist investors, which include the hedge fund Elliott, are unhappy with the proposed £44 per share deal that was announced last November.
They believe the other stock and cash option, technically available to all shareholders, favours the two major investors of Colombia's Santo Domingo family and cigarette maker Altria.
Although there is another option, which involves a mix of mostly unlisted AB InBev shares (at around £39 a pop) and a small cash payout, the stock on offer cannot be sold for five years and means most institutional investors must opt for the all-cash option instead.
In addition, since the UK's vote for Brexit in last month's referendum, the stock-and-cash deal is now far more lucrative, worth around £50-a-share, which has riled many of SABMiller's other investors.
However, at present the £44 per share cash deal is still only a proposal and could theoretically be changed at a future date.
Before it is waved through in all four markets, AB InBev will not submit a formal proposal, meaning the SABMiller board will not be able to make a formal recommendation on the offer.
The board will be meeting ahead of the British drinks giant's AGM tomorrow. SABMiller declined to comment.