Can you suggest a library where I might find a document backing up Macdonald's claim?
A good question; I'm afraid I don't know and can only answer generally. (I hope I'm not insulting your intelligence, and I fear this is probably just telling you things you already know, but...)
First, from whom was the bribe offered, and when? (According to MacDonald, obviously.) Then: does that other party have papers somewhere, or is that other party documented in some way? Are there local histories of that region from that period, which draw upon primary sources that you can then find in a regional archive? Those would be the ways I'd approach it, for what that's worth.
I can only offer examples from cases that I have seen. The cases in Hamburg were relatively simple because there were so many examples, offered by so many witnesses who often provided lists of property and money that was taken, and often lawsuits followed after the Napoleonic Wars, as people tried to get back the things they'd lost, or money and property that had been extorted from them or illegally confiscated. This was often in the form of "protection money," such as: "Give me X and I won't arrest you and confiscate everything." (Because arrest for smuggling carried a possible death penalty and the guaranteed confiscation of All one's property.)
In other cases one only gets a single witness. For example: the memoirs of young Johann von Borcke, who was in the 1st Lt. BN of the Westphalian army. He was sent to the North Sea coast in mid-1809 to assist the French Douanes with anti-smuggling operations, and he says that he saw French officers enriching themselves at the expense of the poor locals whose goods they confiscated, having falsely decreed them to be "contraband" or "English" goods.* This would remain a singular observation, impossible to confirm, except for the fact that it mirrors so closely the experience of so many people in Hamburg, not too far away, at the same time.
Other examples are much more tenuous. For example: a French regiment of dragoons entered a Pomeranian town in the Winter of 1806-7. Napoleon had ordered them to occupy the town. The local town leaders gathered money and valuables and offered them to the colonel as a bribe, if he would restrain his men and not sack the town. According to the recollection of a man who was a boy in that town at the time, the colonel took the bribe and the dragoons moved on, and the town was spared. (In other words: he took a bribe in order to defy Napoleon's orders.)
That tidbit comes from one of the many local historical publications that were ubiquitous in Germany in the mid-19th century, in this case written by the son of the then-Bürgermeister of the town. The nearby archive (in this case, Prenzlau) - (and my memory is shaky because it's been @8 years or so since I was there) - did have a record of French deserters being found in that town a few days after the alleged incidents, so we know the people in question were in the right place at the right time, and we also know that the town wasn't sacked, when others nearby were, because of other memoirs from those locations. (For example: the correspondence of a woman to her son, verifies that the dragoons hauled girls from hiding and raped them, and that their town was sacked by the dragoons.)
So... the story of the colonel taking a personal bribe to restrain his men from sacking the town is certainly plausible, but to get a "smoking gun" in such cases is always very difficult. However, it does ring true to a certain extent because we see it happening in other places and times, under similar circumstances. For example: on 18 Feb, 1807, in response to uprisings of local Hessian rebels in the vicinity, who were alleged to be receiving assistance from townsfolk, Napoleon ordered the towns of Hersfeld and Eschwege burned, "sixty or more" civilians to be shot, and another @200 to be arrested and transported to France. The French officer on the scene, Gen. Barbot, was offered a bribe by the locals not to carry out the orders. We can't be sure whether he took the bribe or not (there are two recollections, one says Yes, one doesn't mention it). But we do know that Barbot passed the buck to his subordinate, a young Württemberg officer named Lingg, who then came up with a ruse to evacuate the people first, and then to torch a few old buildings, and then to quietly move the people back in, so that nobody was hurt or killed. Did Barbot defy Napoleon's orders because of a bribe? Or because he had moral objections? We can't be sure. We can only know that a bribe was offered, and Napoleon's orders weren't carried out.
Obviously, those are the sorts of details that one includes in a full-length written treatment of the subject, albeit not generally on a brief Forum post such as this.
My apologies for the long detour, and for my inability to answer your specific question about MacDonald, but I thought it was at least appropriate to discuss my method, in light of Kevin's... "concerns"... regarding my honesty and academic rigor.
* Leszczynski [no first name] (ed), Kriegerleben des Johann von Borcke 1806-1815 (Berlin: Mittler & Sohn, 1888) Page 155.