It should work that the decrease in price (if honored) is removed from the last few payments for example:
Original $2000 New $1500
$166.66 for 11 payments $166.66 for 8 payments
$166.74 for 12th and final $166.72 for 9th payment
**Side Note** Affirm may charge interest. So, just a word of caution that while it may seem like a great option that's only IF the interest rate is LESS than your credit card. Some credit cards offer travel protection or points/cash back, too. I'm not discouraging using Affirm, etc. Just want folks to be aware that while you can find 0% interest, it's not always the case. Affirm reports some loans to Experian. Therefore, taking out a loan with them may affect your credit score and failing to pay as agreed can impact your credit score.