Another variable to loan amortization

I am wanting to create a loan amortization table that will first reduce an accumulated interest amount ($2500) but still calculating interest off the original loan amount ($15000). When the payment reduces the $2500 past 0, I want to find a rule that will then allow it to automatically start reducing the $15000 basically turning into a regular amortization schedule.

I need help finding or making a rule to connect that interest only payment and then reduction of the principal.

Attachment of where I am it is provided.

Thanks so much for any help!