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.
Sorry, the value of a 2007 dollar in 1864 would have been 0.080. Darn those typos.