This is not currently possible to do in teller processing as it would require the use of two separate Proc Codes for the loan account; a 'T' for the transfer, and an 'L' or 'X' for the principal only payment.

This could be accomplished however either via member account adjustment to move the funds through a suspense GL, allowing control of the transaction description and the allocation of the funds to the loan.  Alternatively, you could add a secondary transaction description for the transaction to indicate that the cash in and out was for a principal only payment on the loan.