Primary and Secondary insurance, bill patient the difference?

I have a situation here.
Example:
Patient's PRimary: Allows $300 but applied to deductible.
Patient's Secondary: Allows and pays $100
This leaves a balance of $200

We are in-network with both insurances.

In this case, do we bill the patient the difference of $200 or write off the $200.

Our allowed rate is higher than the secondary, who's fee schedule contract do we have to follow?

Primary and Secondary insurance, bill patient the difference?