That's not the only cases you can also finance a trade in balance by selling assets. For instance if you buy more products from the Chinese that they buy from you. Then the Chinese could use their excess dollars to buy stocks or property in the US. In this scenario, the US isn't borrowing anything to finance the trade imbalance.

And if the value of stocks and property grows faster in the US than the Chinese are buying it, then this trade imbalance is even susistainable.

Secondly you don't need reserves or printing to borrow for spending. The dollars the Chinese make on selling goods to the US, they could led those dollars back to Americans to allow them buy more Chinese products. The dollars from this purchase can be lent out again. You can keep going like that infinitely.

But of course at some point the Chinese might want to actually spend their dollars to buy something they like.