TRM Valuation TPM1

Hi experts,
Iam tryingassessmentssecurities products, andhave the followingproblems:
1. Purchaseis madeU.M.10,000(FTR_CREATE)
2.The first dayof the month followingcoupon paid500UM(FWSO)
3.I willvaluethelastday of the monthandthesystem givesme the newUMNominal9,500. (TPM1)
The field thatI needto stayis the "Nominal" because,Nominalvaluation isbyits market price, but the system is decreasingmepay principalproduct of thematurity ofa coupon(FWSO).
This I cancheckinTPM12transaction.

If anyone has anyidea how tostoprepaymentswithouteffects on theNominalIcan work withany indication.
Best regards,
ManuelBarrera