The word translated as "the bank" is uncommon for Jesus and a bit of a pun. It has the primary meaning of "table" or "counter" referring to the "tables" or "counters" of money-brokers. This is precisely how we use the terms "over the counter" to refer to trading stocks through a broker network rather than a stock exchange. It is not the Greek word used elsewhere in the NT translated as "money-changers". This word is used because it is a pun, since the word also means the grinding surface of teeth and the lower mill stone which grinds grain. So in a sense the money brokers are "grinders".
However, the particle word translated as "might" and "could" is important here. There is no exact equivalent in English, but it is used to indicate the possibility of something rather than its certainty. So the master is indicating that the interest was not guaranteed, just a possibility. The chance of gain is worth the risk of loss.