DNS Question

I am the administrator of a Windows 2003 domain.

Users inside of the office cannot access sales.abc.com
Users outside of the office can access sales.abc.com since they are
hitting external DNS and are properly routed to the external website
www.salesmaster.com/abccompany using a "WebHop" tool (external =
DYNDns).

Here is the question:

I need to set a DNS record that points sales.abc.com to the external
site.  I will need to use a series of A and CNAME records, I assume.

Requirements:
Users inside the office, using domain controller's DNS, need to be
redirected to external web site.




Note:  Internal DNS is handled by the domain controller.  External DNS
is handled through 3rd party web tool.