What is the ruling on dealing with checks and the profession of money exchange, given that I am obligated to provide cash to employees and cannot do so except by cashing checks?

It is permissible to deal with a certified and post-dated check, but it is not permissible to sell a post-dated check for less than its face value in cash, because that falls under riba al-fadl (usury of surplus) and riba an-nasi'ah (usury of delay). This transaction is prohibited, whether it is considered the sale of currency for currency with a surplus and delay, or a loan in which the lender stipulates an increase. As for working in currency exchange (sarf), it is permissible if the Shariah guidelines are adhered to, including the immediate exchange (hand-to-hand possession) when exchanging different currencies.