The value of the credit is 30.01. When they use it to buy something else, they pay tax on the item they purchase so they really can buy less than 30.01 worth of stuff with it or they have to cover the difference.
You are over thinking this. You report sales tax on SALES. It isn’t a balance sheet for accounting and is calculated for the price at time of purchase.
The item he gave you is worth $50 so you give him $50 credit. If he buys a $19.99 item, the total is $21.74 he must pay out of his credit. He then has $28.26 left over for credit and you reported the $1.75 sales tax for that item that was sold for $19.99. The item you took from him has no value to NY sales tax unless you resell it. If you decide to list it for $100, then when someone buys it you need to report your $100 item and the $8.75 in sales tax for that said item. The credit balance the person has means nothing. If they come buy something else at your store, then they will use that balance to pay the sales tax at time of purchase so if he buys another 19.99 watch band, he will pay 21.74 with his credit and you report the $1.75 sales tax on the quarter the sales was made.