Hi Jandoval,
I agree with you about this, but I don't think that SAP will change this, at least I don't know how to do.
What you can do is manually insert the first repayment with the amount fixed and then create a new one and let the system calculate the rest of the repayments.
I know that is not the best solution but is the only one that I can think of.
Best regards
George Passini