Analysis: Here, Dylan is making a rather obvious "dick joke" as he offers his phallus as collateral. This suggests that his cock is of unusual size or shape, & therefore worthy of the money he needs. There are also homoerotic undertones here as he may be implying he will have sex w/ the banker in order to spring his friends... they get sprung after he gets sprung... as it were. Of course, he's also prostituting himself here, trading sex for the release of his seamen. In general, the song is a critique of European expansion & the dissemination of Western hegemonic forces throughout the new world. Here, of course, we have the play on words that Derrida suggested later (in his book Dissemination) regarding "dissemination": the spreading of ideas & Semen: the spreading out of male power. However, but involving sailors in he mix, Dylan is also able to add the word "seamen" to this fun linguistic game.
Other reading of this line may suggest that he's actually trading his pants, not his phallus for their release. If this is the case, he is willing to sell the very clothes he is wearing top save his friends; however, it seems as though he would need to take them off to accomplish this, not just down as the lyric suggests.