I can't think of a specific reason why you could not set the terms as you and the customer agree. But, if this is an individual (and I assume that it is, or else the loan and deposit interest would offset for tax purposes) wanting to complete this transaction, I have to seriously question his motivation. Are you certain this is kosher - no money laundering or government fraud going on?
You say that it's very sizable - sizable enough to impact your reported loan yields and deposit costs?