There are many ways to approach this depending on what you are trying to accomplish. My initial impression based on the information provided is the following [I am also assuming you have some scripting knowledge (assuming is a bad thing!)]:
First create a script that determines what the payment type is and then sets the correct field to the value found in Total: (in your example 200).
YourDropDownFieldName = "CREDIT"; Set Field (YourCREDITFieldName; YourTotalFieldName);
YourDropDownFieldName= "CASH"; Set Field (YourCASHFieldName; YourTotalFieldName);
YourDropDownFieldName= "CHECK"; Set Field (YourCHECKFieldName; YourTotalFieldName);
Set a script to trigger whenever YourDropDownFieldName is modified. In Layout Mode click on the YourDropDownFieldName then from the menu bar Format: Set Script Triggers: OnObjectModify and select the script you created above. This process will just copy it the appropriate field.
If you really want to move the Total you could then adjust the script to blank the YourTotalFieldName by Set Field (YourTotalFieldName,""). But since we don't have context, the latter may not be a good idea.
These three fields could also be calculation fields that use a case or If function to either be empty or repeat the value from Total depending on the value selected in the field with the value list.