The official purpose of Dahlgren's raid was to free prisoners from Libby Prison, period. None of Dahlgren's superiors or any of his staff were aware of any plan or issued any orders to burn Richmond and kill Davis and his cabinet. Some sort of proclamation stating that goal was supposed to have been found on Dahlgren's body, allegedly in his false leg, but even then there was nothing that indicated that the Union Army command authorized such actions, much less that Abraham Lincoln personally ordered it. There is nothing in the OR to indicate that the assassination of Davis, or even his capture was ordered. Yet you have no problem stating with perfect certainty that Lincoln ordered the murder of Jefferson Davis, so that somehow makes Booth's actions acceptable.
Well, Civil War Times is a very respected publication. If they think it is so, I think there is a good chance it happened.
OFFICERS AND MEN:Dahlgren confided his secret orders to Capt. John McEntee (see The Secret War for the Union). Gen Ben. F. Bultler has proposed a similar scheme which Lincoln & Stanton approved but failed in execution. Seeing the river too high, Dahlgren hung his guide, a black freedman in a fit of rage. Dahlgren's notebook included, 'a set of notations referring to planning for the raid and for carrying it out, including the stark direction: "Jeff Davis and Cabinet must be killed on the spot."'*
You have been selected from brigades and regiments as a picked command to attempt a desperate undertaking—an undertaking which, if successful, will write your names on the hearts of your countrymen in letters that can never be erased, and which will cause the prayers of our fellow-soldiers now confined in loathsome prisons to follow you and yours wherever you may go. We hope to release the prisoners from Belle Island first, and having seen them fairly started, we will cross the James River into Richmond, destroying the bridges after us and exhorting the released prisoners to destroy and burn the hateful city; and do not allow the rebel leader Davis and his traitorous crew to escape. ...Guides.—Pioneers (with oakum, turpentine, and torpedoes), signal officer, quartermaster, commissary. Scouts and pickets. Men in rebel uniform. These will remain on the north bank and move down with the force on the south bank, not getting ahead of them, and if the communication can be kept up without giving an alarm it must be done; but everything depends upon a surprise, and no one must be allowed to pass ahead of the column. Information must be gathered in regard to the crossings of the river, so that should we be repulsed on the south side we will know where to recross at the nearest point. All mills must be burned and the canal destroyed, and also everything which can be used by the rebels must be destroyed, including the boats on the river. Should a ferry-boat be seized and can be worked, have it moved down. Keep the force on the south side posted of any important movement of the enemy, and in case of danger some of the scouts must swim the river and bring us information. As we approach the city the party must take great care that they do not get ahead of the other party on the south side, and must conceal themselves and watch our movements. We will try and secure the bridge to the city, 1 mile below Belle Isle, and release the prisoners at the same time. If we do not succeed they must then dash down, and we will try and carry the bridge from each side. When necessary, the men must be filed through the woods and along the river bank. The bridges once secured, and the prisoners loose and over the river, the bridges will be secured and the city destroyed. The men must keep together and well in hand, and once in the city it must be destroyed and Jeff. Davis and cabinet killed. Pioneers will go along with combustible material. The officer must use his discretion about the time of assisting us. Horses and cattle which we do not need immediately must be shot rather than left. Everything on the canal and elsewhere of service to the rebels must be destroyed.
The War of The Rebellion: A Compilation of the Official Records of the Union and Confederate Armies, Washington DC: Government Printing Office (1891), Series 1, Vol. 33, pp. 219-220 (similar on p. 178 and 179)
* For a detailed analysis, see The Dahlgren Papers Revisited by Stephen W. Sears