Could you imagine getting audited and your boss finding out you donated $xxxx dollars in donated patronage to others? I would not want to be the one in that room...
As with any expense, you should have a good business reason for it. But I think there's a strong business case for businesses supporting open source software that's key to that business.
Either way, it's $20 on some vague software expense. Nobody will care. Worst case, the boss somehow says, "What's this $20 charge" and I explain why I though there was a good business reason for us to support that project.
If you said the money is going to GitHub, and it's actually going through GitHub, that's lying.
I think it will probably show up differently on credit card statements anyway, so people won't be tempted to do this behind the backs of the accounting department.