Refundable deposits are best represented as Products. You can also assign them to dedicated deposit financial accounts. In your accrual reports, you will also see how much liable deposit revenue you have based on how many deposit products you have sold but not yet refunded. This product once created should be added to the sign-up fees tab under the relevant price plan.
First create a product in the Inventory > Products section. Add this product in the sign up fees tab inside the plan. Products added in the sign-up section of the plan will be billed in the first invoice.