When you enter medical expenses in TurboTax, including dental expenses, you do not have to specify who you paid them to. You just enter the amount that you paid.
In your medical expenses you enter the total amount that was paid to the dental company. Include both the amount that you paid yourself and the amount that the finance company paid to the dental company. That is your total medical expense. The amount that the finance company paid is a loan to you. For your medical expense deduction it doesn't matter that some of what you paid was your own money and some was from a loan. It's the same as if you went to a bank and took out a loan to pay for the dental work. You deduct the full amount in the year that it was paid to the dental company.
You cannot deduct your payments to the finance company, this year or in future years. They are not medical expenses. You are just repaying the loan that you used to pay the medical expenses.
