Need help with possibly slapped site.

I had a client that came from a SEO company that was pretty "blackhat", he got up to page 1 pretty quickly. After I started doing some basic clean link building penguin came out and his rankings tanked. Basically his sub pages are now ranking higher, and that is on page 3-4 even. His root domain went into oblivion. Any suggestions on fixing this site? Don't really want to go through redirects and all that shit since this is an ecomerece site. He is also a low level client $300 per month, so don't have insane amounts of time to dedicate to it. Key doesn't seem all the competitive based on first page results.

You can collect all the bad links which are pointing to the domain and submit them to disavow tool. I don't think of any other solution apart from building more quality links to the domain. Its always difficult to get back from Penguin.

First of all you need to know exactly what happend before trying to find a solution.

If you go into the Google Webmaster Tools for that site and see if they have anything listed in there. If not, send a reconsideration request and see what they have to say as at least you will know one way ot another if they have slapped it. Once you know this you will then have a better picture of what steps to take next.