Payment Schedule - Installment Amt vs Balance Due

Hello Everyone, 

Considering all things, we are in a new situation where we are honoring monies from a cancelled performance into subscription packages for next season.

A good chunk of our guests who are requesting the funds go toward their package for next season are currently on a payment plan, which is where things get a little confusing. 

If a guest has a remaining balance on their order of let's say $200 but the final installment is $500 which one will charge??

We have already moved the funds out of the cancelled performance and into the package order for next season, but with a payment schedule in place I'm concerned about overcharging guests if the installment amount charges versus the actual balance due on the order. 

Any insight would be fantastic. 

Thank you!

Nadinne