You would have to charge them and then do a refund - that may not be the best option for your customer as refunds back to cards can take up to 30 days. To shorten the deposit refund time, one option would be to use the Send Money feature but, using this method would require the customer or client to have a PayPal Account. One thing to be aware of is how the system looks at refunds - for example, you're in the rental business and refunding deposits are an everyday thing - the PayPal system is really not designed for that type of function, so it may flag your account due to the frequency of refunds. It's not that you're doing anything wrong but, the system may see the transactions as a red flag.
PayPal does have another feature that could possibly work, it's called authorization and capture, it can be useful in certain situations - just something to checkout.
