Hello Ainura.
Take a look my answer in this topic SAP Treasury - How to create taxes for Issuer
For your case you can use Derived Flows.
with 1a. Define Derivation Procedures and Rules you can tell reward flow to take amount of the transfer as the basis for calculation of reward amount. Everytime you make a transfer of the tranche, reward flow will appear.
If your percenteage of reward is dependent on the basis amount - use can use 1b. Define Calculation Procedure for Derived Flows if you don't have it, don't use it.