According to my reading of the OED, the terms were interchangeable during the late 18th century, with the "farm produce" meaning becoming more common through the 19th century.

I am sorry to not be able to contribute more - perhaps a historian of the 18th century South, or of slavery, will be able to do better. Given the economic history of the slaveholding regions, which heavily privileged raw agricultural products (principally tobacco and rice during the 18th century, with cotton increasingly ascendant, as well as other cash crops like indigo and sugar), we might surmise that these were more likely. I suspect, however, that that does not preclude someone trading something like barrels of nails or other manufactured goods entirely. Probably not that likely, however.

I'm not sure myself about the interchangeability of the terms in the 18th century, but otherwise /u/pipkin42 is basically right - there were definitely some slave traders who would explicitly market their captives as being for sale for "cash or cotton" by the early 19th century (likewise for other commercial goods, services or even settling accounts of credit), and I rather suspect "produce" in this context referred to cotton and potentially other cash crops, probably tobacco. Unfortunately, our record of this particular slave voyage - like most - is very partial; we know that 169 saves arrived on May 7 from the Windward Coast (with 23 more dying during the journey) and that they were sold on, but not for how much.

In the case of the slave trade, there could be two motives for accepting cotton in lieu of cash. The first is that it is obviously a very good substitute for cash being a good in high demand with a strong market price; a slave trader obviously had the means to sell it on to the right parties with ease. The second is that individual slave traders may well have had their own industrial or commercial interests. If a slave trader had investment in or ownership of a cotton mill back in Britain, they may have found it more cost-effective to take cotton in exchange for slaves rather than paying market prices for the raw material itself.

I don't imagine you would have had many traders at all accepting manufactured goods or equipment though. Slaves were an incredibly valuable commodity and cash crops were a good substitute only because they were easily exchangeable for cash or otherwise useful as raw materials; I have a hard time imagining an exchange for something like furniture ever being worth enough even for an average slave, given the ever-growing demand fresh labour (especially after the advent of the cotton gin). Having said that, it isn't totally inconceivable that an arrangement like that could have been reached as part of a wider payment plan in some individual cases. Someone more familiar with eighteenth century commerce beyond slavery might have a better understanding of the potential there.