Consider the following scenario in Microsoft Dynamics SL:
In Service Dispatch in Service Call Entry (SD.200), enter in a new Service Call for any customer and Save.
Open the Invoice button at the bottom and enter any required fields in Invoice Entry (SD.202) screen and Save.
Open Invoice T&M Details (SD.203) and enter a Billable line for an Inventory Item (item would be non-stock and user specified cost).
Enter a Quantity and Unit Cost and tab through the Service Date. You will notice the Unit Cost is reset to zero.
To resolve this problem, customize the screen and move the Service Date to the position before the Unit Cost and Unit Cost will keep its value.