Here is one way that seems to work.

Assumptions:

- G1:G4 contain the payments terms
- H1:H4 contain the amount, so H1 would have the start amount, H2 would hold the increased amount. If you wanted to increase the amount after two payments put it in H3, not H2.
- D1: D4 contain the payments due.

Use thse formulae
D1 is simply 
=IF(H1,H1*$G1,"")
D2 is 
=IF(H2="",D1,ROUND((H2-SUM($D$1:$D1))*$G2/SUM($G2:$G$4),2))
Copy D2 down to D3 and D4