This will work reliably only because you share an upstream provider, who will presumably be passing on your announcement of your shorter prefix to the rest of the Net, and who can presumably be paid to listen to the /24 anouncements. A further complication may arise if you have another upstream, who either isn't listening to the /24 announcements or has peers who aren't listening to them. Traffic to your fomer customer could end up taking a rather roundabout route, either through your other upstream or through your network.
If you are in the case where some of your former customer's traffic may pass through your network, you are entirely justified in charging the customer a fee for this 'service'. You may get some of his inbound traffic unless all of your upstreams accept his /24s and meet with Sprint directly.
The weird routing scenario I described above (which depending on who your other upstreams are may not happen) would not only cost you money for bandwidth to carry traffic that you presumably aren't being paid for anymore, but would also have performance issues and may run into anti-spoofing filters, which would have to be modified.
The problem is, if your Sprint connection isn't live, you will have no sure way to get his traffic to him, and that's the problem. If you receive some of his traffic from upstreams of yours that don't accept his /24 advertisements, you will have to get them to him in some way. You will simply need to find one route to him that works, and push all his traffic out that way. What will stop this from working are: 1) If the upstream just hands the traffic back to you because it doesn't see your customer's /24s. 2) If the upstream or something on the path does see your customer's /24s and decides that you shouldn't be originating the traffic and so blocks it. It's theoretically possible that all of your upstreams will have one or both of these problems, and hence you will have no way to get traffic to your former customer. Worst case, your former customer can always get a few IP address from another provider (router interface addresses are fine) and set up a tunnel to you. That will allow you to get him his traffic if it winds up on your doorstep. Unfortunately, many tunnelling techniques aren't truly transparent, and may not support a payload MTU of 1500. :( Ugly, ugly, ugly. DS