[A_Brandybuck]

I think, that the behaviour of Lotho is crucial in this discussion.

Return of the King, The Scouring of the Shire:
He'd funny ideas had Pimple. Seems he wanted to own everything himself, and then order other folk about. It soon came out that he already did own a sight more than was good for him; and he was always grabbing more, though where he got the money was a mystery: mills and malt-houses and inns, and farms, and leaf-plantations. He'd already bought Sandyman's mill before he came to Bag End, seemingly.
'Of course he started with a lot of property in the Southfarthing which he had from his dad; and it seems he'd been selling a lot o' the best leaf, and sending it away quietly for a year or two. But at the end o' last year he began sending away loads of stuff, not only leaf. Things began to get short, and winter coming on, too. Folk got angry, but he had his answer.

Farmer Cotton clearly stated, that there is money in the Shire. At least Lotho bought some buildings with money. The question came up, where he got all the money and it seems clear, that he had some tradings with Isengard, which is (for me) obviously the source of his money.
Farmer Cotton didn't know, where the money came from, so I think, that this was very 'normal' money, which seems to look not very exotic. I mean Farmer Cotton didn't explicitly mention that this was money from elsewhere or that there was such a rumour. In this case he would clearly have mentioned it, because this had (at least partly) answered the question, where he got that money.

And alone the possibility the Lotho could make such deals with some foreigners could mean, that there must be a shared basis, of which the tradings underlie.