I think that the deposit represents a Liability to the company, they must recognize that this money is not theirs on their financials.

However, what they do with the cash in the meantime is up to that business. Only should they fail to return the deposit is that a breach of contract. Unless the institution is a externally regulated one - for example a bank - then the regulation from financial industry apply.

And they do not necessarily have to give you interest unless it was on the contract. Always refer to the fine print with interest and refer to the type of regulation applicable to that industry.