Device Promotional Credit applied to Asset Liability

The business entered into a 36 month payment plan for tablets from the service provider.

A liability account was created with the total amount due.

The monthly payment for the tablets is expensed to the liability account.

BUT the monthly "Device Promotional Credit" is a problem.

I need to reduce the original cost of the Fixed Asset and I need to reduce the liability cost of the loan.

When I itemized the monthly bill, enter the credit from the bill as a negative amount and expense it to the liability account it shows on the register as ADDING to the overall total rather than decreasing it.

When I itemized the monthly bill, enter the credit from the bill as a negative amount and expense it to the original cost of the Fixed Asset, it enters correctly and brings down the total original cost of the asset, but I don't know how what account to use to balance a reduction to the asset liability.

I could not apply a vendor credit to a liability account.