Because Alexander Hamilton wanted the federal government to assume the debts of the states and Jefferson disagreed ... until Hamilton offered his support for a federal capital on Virginia's Northern border.
Favor trading has been a part of our politics from the beginning. You only call it bribery when someone you disapprove of does it.
I call paying people in exchange for their votes bribery. I provided the definition of bribery given in an 1856 dictionary of US law terms, and it seems to fit this particular case. On the other hand, you term this sort of transaction "favor trading."
Let's calculate the value in present day dollars of one of those three jobs that were offered in exchange for three votes, the one mentioned at $20,000 a year. Using an Excel inflation spreadsheet available on the web [Link], the value of a 2007 dollar in 1864 would have been 0.80. In other words, an $20,000 job in 1864 would have been worth $250,000 a year in 2007.
Some favor.
In searching I found that Stanton took his cabinet post under Lincoln at a salary of $8,000 [Link 1]. And I found customs regulations of 1864 that limited the aggregate of salaries, fees and commissions to a maximum of $2,500 [Link 2]. Any excess was to be turned in to the Treasury Department.
So, the $20,000 salary quoted by Dana sounds suspiciously high. Perhaps there was a typo and the figure should have been 2,000. Or, perhaps a crooked official could really earn the $20,000 a year in fees, etc. and just not turn over the excess to the Treasury. Who knows?
From the speech of Representative C. H. Van Wyck of New York to the House of Representatives, February 23, 1863, starting with the headings [page 117 of Appendix to the Congressional Globe for that session, 37th Congress, third session, Link]. Van Wyck was making a minority report about what he had found out about the true earnings of customs officials from interviewing 50 of them:
FRAUD ON THE TREASURY -- THE NEW YORK CUSTOMS-HOUSE
... MONEYS RECEIVED BY REVENUE OFFICERS
There was a difference of opinion among witnesses as to the amount of moneys received by the revenue officers. Mr. Cargill, a very candid man. formerly in the custom-house under Schell, as deputy, in a position which entitled him to know, states it at $150,000 to each for four years, independent of the salary. Also, Mr. Bayard, then and now in the seizure bureau of the customs-house, a cautious, prudent witness whose office gave him the means of knowing, and who had previously made calculations as to the amount, fixes it at $30,000 per annum to each ... The figures as made by Mr. Ogden [rb: an auditor] show from fines penalties and forfeitures, between twenty one and twenty two thousand dollars per annum, besides the salary of $5,000.
In addition the collector receives from commissions on the harbor master, health officers, and seaman's retreat hospital fees, about four thousand dollars more, making according to their own figures about thirty thousand dollars per annum for the collector, and $27,000 each to the surveyor and naval officer.
Possibly there should be no censure for the officers who take these fabulous amounts when the law gives it to them, but what outrage and enormity in the law which can tolerate such a thing! A party in power which creates or continues such a law ought to be rebuked. If these gentlemen are worth $30,000 a year to themselves, they are certainly not to the government. Without any disparagement to them, who will pretend that their places could not be filled with men equally competent for the salary of $5,000.
According to Van Wyck, the figures above do not include bribes. No wonder that Lincoln was besieged by patronage job applicants, and Dana could offer such a job in exchange for a vote. Van Wyck said that his colleagues on the committee that investigated this were trying to bury the information, hence the need for Van Wyck's minority report.
I gather from Van Wyck's words that this customs bonanza had gone on during the Buchanan administration as well and in fact for some 20 years, at least. So, you are correct that this kind of operation was the modus operandi back then. All this probably led to the later limitation on customs officials' earnings.
By the way, the average salary of a working man in 1860 was about $290 per year according to the census. [Link].