Is handing over a sum of money to a cashier without taking a receipt considered a sin or an iniquity, especially if there is a later suspicion of misappropriation of the amount by the cashier, and should one leave their job due to this suspicion?

If your job requires you to deliver the amount with a receipt, then you have erred by delivering it without one, and perhaps the cashier took the amount unjustly. However, if a receipt was not necessary, then there is no sin upon you. To be at ease and clear your conscience, inform the company owners about what happened.