Experiences with air travel CO2 emissions offsetting funding rules?
In general, grant or contract money cannot be spent on anything except the designated purposes permitted by the funding agency.  If the funding is coming from an agency of the US Government, those purposes probably are not allowed to include carbon offsets.

That said, it does appear that at least one university has figured out a way to pay for carbon emissions.  The key seems to be imposing the cost as part of departmental overhead, such that it's not directly associated with any grant, but mixed in with all the other things the university supports and the funders have no control over, like pencils, daycare benefits, on-the-job injury insurance, and deans of student happiness.