Product: Payday loan, title loan, or personal loan

On XX/XX/2018, I signed a contract with XXXX XXXX XXXX XXXX to replace two front doors of my home. 
The contract was for a total of {$9400.00}. At the time of signing the contract, I immediately paid {$5000.00} and left an outstanding balance of {$4400.00}. 
XXXX XXXX offered me a promotion to pay the balance in 6 months without interest or penalties. I accepted that facility, but at that time XXXX XXXX did not give me any details of how that facility would work. 
On XX/XX/2018 I received an email from XXXX XXXX informing me about the person who would be the Coordinator of my project, the instructions for the day of the installation of my project and the instructions for the final payment. 
For the final payment, XXXX XXXX informed me that I would have to pay the total balance through a personal check or through a financing with XXXX or XXXX ( I interpreted that they were financial firms that XXXX XXXX uses ). 
In the meantime, I received an email directly from a GreenSky company, unknown to me, mentioning my operation with XXXX XXXX and offering me a line of credit for {$10000.00}. This GreenSky company even already assigned me an account number to activate the line. 
I was confused. I did not know this GreenSky company, nobody in XXXX XXXX had informed me and it was not either of the two companies ( XXXX or XXXX ) that appeared in the final payment instructions sent to me, in writing, by XXXX XXXX on XX/XX/2018. 
On XX/XX/2018 I wrote an email to XXXX XXXX asking them to inform me who GreenSky was, because I was offered a line of credit and what was the relationship of that firm with XXXX or XXXX. No one from XXXX XXXX answered anything. On XX/XX/2018, I wrote back to XXXX XXXX an email asking about the payment method of the final balance and nobody answered me. 
On XX/XX/2018 XXXX XXXX installed the doors in my house and that same day the Installation Team Leader, at the end of the work, requested the final payment that I made with a personal check following the instructions, in writing, that I received from XXXX XXXX on XX/XX/2018. 
At no time did the XXXX XXXX Installation Team mention or inform me of any type of financing in my project for anyone, therefore I had to pay the entire balance. 
After the installation of the doors and my final payment, I was telephoned by an official of the XXXX XXXX Financial Department to inform me that in my project they had " two final payments for the same amount of {$4400.00} ''. One that corresponded to a personal check of mine and another paid " in my name '' by GreenSky. I requested that my personal check be used since I had never activated or authorized any transaction with GreenSky. 
On XX/XX/2018, GreenSky sent me a receipt to pay {$51.00} for " Interest Charged for this Statement '' (? ). It strikes me that I must pay interest for an operation that never activates, authorizes, or disposed of a penny and that according to the " offer '' that " sold '' XXXX XXXX was exempt from interest and penalties. 
It seems totally unfair to me that : ( a ) a firm like XXXX XXXX offers me a financing facility and does not correctly inform me how it works ; and ( b ) the GreenSky company wants me to pay {$51.00} in interest for an operation that was not activated, authorized and not even a penny was used.