here, the bank's assets increase by the value of the T-bill and liabilities increase by the amount that they paid you for it. in the balance sheet the T-bill will have the same value as what the bank paid you/deposited into your account
the bank doesn't need to have the money to pay you for the T-bill, as that money will be created and deposited into your account on the bank's sheet side it works: its assets and liabilities increase in the same amount