What is the ruling on dealing with metal currency that merchants refuse to accept except with a 2% increase, which leads to freezing capital and inability to purchase necessities?

The increase mentioned in the question is an increase in the price of the sold item (manufacturing supplies), not an increase in selling metallic currency for paper currency. Therefore, there is no objection to this sale. If the transaction were the sale of metallic currency for paper currency, some contemporary scholars have permitted the difference in amount, arguing for a difference in kind, and citing the Prophet's (peace be upon him) saying: "If these types differ, then sell as you wish, provided it is hand to hand."