Increase the send limit of the group to 15000. If you have more than one customer in that group you will need to create another group with limit of 15000 and assign the customer to it. You can copy a group and just change limit whilst maintaining all other settings
Don't you think, this is not best solution. Let's say, I have a client, who has used 9002 out of 10000 and he buys again additional 10000 then I have to create a new group with 10998 and then put him on that group, its bit exhausting.
We should have a feature to add additional credits.