Experiences with air travel CO2 emissions offsetting funding rules? The human-induced climate crisis happening right now is no longer news. While many people urge their governments to change policies to reduce our carbon footprints more easily, there is also the possibility of individual action, or change - such as less frequent flying.

In academia, flight can be reduced by some amount (decentralized conferences, etc.) but to many people it seems difficult if not detrimental to science or mankind in total to stop flying altogether.

It is possible to offset the CO2 emissions of a flight by spending money on some carbon capture project or some project that reduces future emissions (e.g. by renewing technical machinery in third world countries). Where does this money come from? I know from colleagues that they asked their university administration to use their travel funding for some CO2-offsetting agency, and were told that this was not possible.

If I understand the rules in Germany correctly, it is not the funding body who makes such a decision but the states general travel rules for state employees.

The question I have is:

Does anyone have experience in negotiating (with a positive outcome or not) such a use of academic travel funds for offsetting the carbon emissions? If so, how did it go? What were the stumbling blocks?

If you thought about the issue but had reservations on why this might not be a good idea at all or why it is hard to actually implement, I would also appreciate your comments, though I'd prefer if you could phrase it as answer to the question(s) stated.
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.