> The phrase "nullify a share grant" has no legal meaning that I'm aware of.

I think it was used more as a description of the effect of the acquisition agreement. And it is not uncommon to structure these agreements as an 'asset purchase' where the there is money exchanged for the 'good' assets of a company, and the shell is left behind holding the 'bad' or 'toxic' assets. In those agreements the acquiring company pays the shell company some cash for the assets, which the board of the company being acquired can generally decide to disburse in any way they choose, but they will generally do it in a way that minimizes legal exposure, so pay of outstanding loans, fees for breaking leases, contracts, Etc. and then the shell dissolves.

In that process all sorts of things can and do happen, and it depends on part on the corporate bylaws and the laws of the state in which they are incorporated.

That said, RSUs also have a notion of 'vesting' and prior to vesting are not available to the grantee. If you redefined the RSU program to never vest that would essentially 'nullify' the RSUs or if you simply cancelled them outright.

Vested RSUs are called "stock" :-) and the class of shares can have its own set of voting and liquidation rights.

Bottom line is that while there are 'common practices' the rules are pretty flexible. The major flex points are when the company is incorporated, when a stock pool is created (as it would be for RSUs and ISOs etc), and when a company is acquired.