Sometime early this year, Apple announced to explore the possibility of offering partial refunds to select customers who purchased a replacement battery prior to the launch of its discounted battery replacement program.
On Wednesday, Apple announced a credit for out-of-warranty iPhone battery replacements.

Customers who paid for an out-of-warranty battery replacement for the iPhone 6 or later devices between January 1, 2017, and December 28, 2017, may be eligible to receive a $50 credit. Apple defines eligible battery replacements as those that were completed at an authorized Apple service location – either at an Apple Store, an Apple Repair Center or an Apple Authorized Service Provider.

Apple says eligible customers will be contacted via e-mail between May 23, 2018, to July 27, 2018, with instructions on how to obtain the credit. If you believe you are eligible for a credit but don’t hear from Apple by August 1, 2018, you’re encouraged to contact Apple regarding the matter. Note that you may have to provide proof of purchase to secure the refund.

According to source, the said credit will arrive as an electronic funds transfer or a credit on the credit card used to pay for the battery replacement.
In-warranty iPhone battery replacement customers are not eligible for the credit, Apple says.

Apple announced a price reduction for out-of-warranty battery replacements on the iPhone 6 and newer late last year, slashing the price down to $29 following a major controversy over CPU throttling.