Was bribery instrumental to the approval of the Thirteenth Amendment?

Question: Was bribery instrumental to the approval of the Thirteenth Amendment?...

Yes bribery was instrumental, and not just bribery with patronage jobs.

Pulitzer Prize winning author David Herbert Donald in his biography entitled Lincoln (page 554) quotes Thaddeus Stevens the leader of the Radical Republican faction as saying

"the greatest measure of the nineteenth century was passed by corruption aided and abetted by the purest man in America"

I started out upon the theme that giving patronage jobs wasn't legally the same as bribery. Upon researching the facts, it seems the movie's political correctness understates the bribery which occurred in passing the 13th Amendment. The primary historical objection to the events in the movie from my research wasn't whether malfeasance occurred but that the movie placed Lincoln's fingerprints directly on said malfeasance, it even had Lincoln taking part in negotiations with the congressmen. Several accounts I read stated Lincoln was too shrewd and careful a politician to have his fingerprints anywhere near the sausage making which was interfacing with Congress in 1865. History however is vague about Lincoln's direct involvement beyond saying "any means necessary" to his secretary of state Seward in order to obtain the votes. To Seward, any means necessary did include patronage and outright bribes. The amendment passed as the movie depicts during a lame duck congress ([the amendment passed 119-56, a two-vote margin][]), with sixteen Democrats abstaining all but two lame ducks. source History also shows patronage jobs even an ambassadorship not discussed in the movie going to those lame duck congressmen who voted in favor or abstained. I also found at least one source stating Seward made use of a "large fund" earmarked for bribes in his efforts.

• ambassadorship to Denmark

  Congress passes the 13th amendment 150 years ago today
  The administration took advantage of the timing of the lame-duck Congress by offering patronage jobs—and in one case an ambassadorship to Denmark—to defeated Democrats.

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• Any means necessary!
• a great quote from the leader of the Radical Republicans Thaddeus Stevens
• Seward's "large fund for bribes"

  Thirteenth Amendment to the United States Constitution
  Lincoln instructed Secretary of State William H. Seward, Representative John B. Alley and others to procure votes by any means necessary, and they promised government posts and campaign contributions to outgoing Democrats willing to switch sides. Seward had a large fund for direct bribes. Ashley, who reintroduced the measure into the House, also lobbied several Democrats to vote in favor of the measure. Representative Thaddeus Stevens later commented that "the greatest measure of the nineteenth century was passed by corruption aided and abetted by the purest man in America"; however, Lincoln's precise role in making deals for votes remains unknown.

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• Lincoln didn't "directly" bribe congressmen.

  Fact-Checking 'Lincoln': Lincoln's Mostly Realistic; His Advisers Aren't
  Sean Wilentz, one of those rare historians who moves seamlessly between the academy and the public sphere, noted that "Abraham Lincoln was, first and foremost, a politician." Lincoln probably didn't bribe congressmen to pass the 13th Amendment, but he instructed others to do so.

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Abraham Lincoln and the two 13th Amendments
According to New York Times commentator David Brooks, Lincoln to achieve his goals, “feels compelled to ignore court decisions, dole out patronage, play legalistic games, deceive his supporters… The movie shows a character-building trajectory, common among great politicians, which you might call the trajectory from the Gettysburg Address to the Second Inaugural.” And therein lies the film’s failing to demarcate Lincoln’s diametric journey between his two 13th Amendment.

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Congress passes the 13th amendment 150 years ago today
As the 2012 Steven Spielberg biopic “Lincoln” portrayed, the president and Secretary of State William Seward were willing to strong-arm border Unionists and horse-trade with reluctant Democrats to secure their votes or at least their abstentions in order to lower the threshold for a two-thirds majority. The administration took advantage of the timing of the lame-duck Congress by offering patronage jobs—and in one case an ambassadorship to Denmark—to defeated Democrats.

Sources:

• Fact-Checking 'Lincoln': Lincoln's Mostly Realistic; His Advisers Aren't
• Abraham Lincoln and the two 13th Amendments
• Congress passes the 13th amendment 150 years ago today
• How True is Lincoln
• Lincoln and the Politics of Slavery: The Other Thirteenth Amendment and the Struggle to Save the Union
• The Thirteenth Amendment to the United States Constitution

I'm not very familiar with this history but some quick research suggests that the movie greatly exaggerates the historical facts.

A prominent historian discusses the historical veracity of Lincoln in The Atlantic and states that, "Lincoln probably didn't bribe congressmen to pass the 13th Amendment, but he instructed others to do so." Unfortunately this particular article does not elaborate the point further or provide any sources.

Consistent with this but similarly brief, an article in the New Yorker about the life of William H. Seward states that "Lincoln and Seward [...] offered political positions to editors who supported [the Amendment] and Seward hired disreputable lobbyists to secure the votes of resistant Democrats and ambivalent border-state Unionists."

Here is a blog entry (also by a historian) which identifies some details of the movie which are entirely fictional:

The movie makers invent a series of quick scenes involving fictional congressmen and the bribes that it takes to sway them. The most notable example of this corruption involves Rep. Clay Hawkins of Ohio (Walton Goggins) who Bilbo / Spader initially switched with the promise of a postmastership in Millersburg, Ohio. The movie actually has President Lincoln himself commenting cynically on this news by remarking, “He’s selling himself cheap, ain’t he?” (Scene 13). All of this is made up.

I found a book, written by Brown professor Michael Vorenberg, called Final Freedom: The Civil War, the Abolition of Slavery, and the Thirteenth Amendment (Cambridge University Press), which discusses the issue on pages 198-203, citing several contemporary sources.

The author mentions a few exchanges made for votes, like appointing the brother of a congressman to a certain position. But he remarks that

There is not one reliable source, nor even an unreliable one, that reports the president making any specific promise in exchange for a vote for the amendment.

He then reviews some statements to the effect that money changed hands, but concludes that

Evidence of bribe taking by anyone voting for the amendment has yet to surface.

So it appears that it was indeed all just politics as usual. Some promises, some favors, some patronage, but no corruption or bribery strictly speaking.