It's true that both sides of the political aisle have had to deal with donations that give the appearance of influence. And the timing of this one certainly raises concerns about objectivity. You can't say it's a quid pro quo though, because Allison is on the giving end of both the favorable report and the money. It's not like he got a contribution in return.
That said, such a hefty contribution immediately following an exoneration of the Obama administration clearly raises questions about the integrity of his findings.