Customer Samples and Charging the appropriate sales rep

What is the best way to charge a sales rep for COG on customer evaluation units/samples?  For example, we have inventory items that were purchased and brought into inventory ($50 for example sake) and then invoiced to Customer X at $174 so we can track pricing.  We wish to credit the client for the eval/samples while charging the sales rep for the COG ($50).  I created a credit inventory item with a COG GL and add that line at "-$174" but this charges the rep for $174, not the true COG ($50) so that  doesn't work. 

the only way i can think to do this is to issue a full credit ($174) after invoice to apply to the original invoice ($174), which brings the goods back into stock ($50) and then adjust the goods out to a COG account and use create a "customer account" that is dedicated to the sales rep, and link the adjustment to that customer account so it shows on the P&L for the sales rep.

is there a better way with less steps?