In order to hold funds the minimum permissive structure would be to get licensed as an escrow company and setup a bank account specifically for the escrow funds that were not part of your operational funds. Most lawyers could provide such a service if it were sufficiently profitable to them. You would need to check into licensing for escrow companies in your local jurisdiction to be compliant. You should certainly not be trying to make any kind of interest or "money on the money" without proper licensing to do so.