I think they can. Just call it a purchase. That's what I do. I have "purchased" licenses from many MIT licensed projects. I intend to do it for every one I put into my project (once I have it figured out). Also, libre office, mozilla, etc. It just goes into the 'software' account.

I don't think the IRS is going to give a shit. Other companies may have shareholders who cry a little, but they could do it, at least to some level. Sending some love to Debian is a better option than paying for a MS license in some cases.

I think that the companies can and should support FOSS more. Especially if they don't want everything to end up GPL'd.