There was a general scarcity of currency in circulation, and a variety of it- states issued their own , and during the time of the Revolution both the individual states and the Continental Congress had borrowed a lot from the Europeans and had also simply printed notes. Soldiers- including Shay- had been paid in Continental currency, which was pretty worthless. After the Revolution, with the Colonies greatly in debt, the Europeans began tightening their credit to Colonial merchants and asked for more hard currency- not Continental, or other similar promissory notes issued by the States.

The Colonies had a very agricultural economy. Farmers make most of their money seasonally: when the harvests come in, when the cider is pressed, etc. Even in the 19th c. there was a great deal of carrying of accounts: the storeowner would have a ledger page with a farmer's name on it, and over time would tally up debts ( a few yards of cloth, a jug of rum) and credits ( five pounds of cheese, a bushel of apples) . The farmer could often keep accounts as well.. At some point, when the farmer sold his crop, the two would settle their accounts. There was also a great deal of passing of personal notes: essentially writing someone an IOU, which they could in turn pass on to someone else to pay a debt, and which could be presented to the debtor for payment. I am not sure about the details of the Rebellion, and what was meant by "cash" in that Massachusetts law, but hard currencies, specie currencies- like Spanish dollars- had risen greatly in value because of the national debts and the economic depression after 1783, and it seems that the rural merchants and farmers were carrying accounts and passing notes much longer rather than trying to settle them. If 1786 law decreed that everything- importantly, taxes which had been levied to pay those state debts- be settled in a hard currency, it would have been very hard on the western economy- if, for example, a bull was worth 5 English shillings in 1783, but only 2 shillings in 1786, demanding the farmer sell for cash in 1786 ( instead of carrying an account listing it as 5 shillings) meant he immediately lost 3 shillings. And of course, if all the farmers had to settle up at the same time, those English shillings became even higher, the market value of a bull lower. The law, therefore, was very punitive to the agricultural economy, supportive of those in international trade: which is why the Boston shipping merchants were the ones who got it passed.

I notice that the Springfield Technical College has assembled a nice representative set of period documents relating to the Rebellion here, including a 1781 posting of the rates of depreciation of the state currency against specie, and an Address to the People by Daniel Pelham, one of Shay's group. Topping the list:

1st: The present expensive mode of collecting debts, which by the reason of the great scarcity of cash, will be necessity fill our gaols with unhappy debtors, and thereby render a reputable body of people incapable of being serviceable either to themselves or the community.

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