@Suitestallion sorry just saw your earlier message
I would add a checkbox to the invoice called “due date moved”
I would make a scheduled workflow based on the Invoice record. The saved search criteria would be
Type = Invoice
Terms =50% Down/50% Net 30
Formula Numeric = {amountpaid}/{amount} >= 50%
Due Date Moved = F
Then the state of your workflow has 2 actions
1. Set Field Value action on Entry, Field = Due Date, Value={today} + 30
2. Set Field Vlaue action on Entry, Field = Due Date Moved, Value = T
Then if you had multiple terms you could always make a few Set field value actions to move the due date
For example, set a condition of terms = 50% Down/50% Net 45 then Set field value to Due Date + 45
And if terms are 50% Down/50% Net 15 the set field value to Due Date+15
Also could you just make 2 invoice one Due on Recipt and 1 Net 30?