"Is the listing fee credit amount the original listing fee (promo) or the relist fee (regular fee) ? "
It depends where the item is listed.
eBay.
com is very specific: the relist credit will be the
lowest of the two. Check this http://pages.ebay.com/help/sell/insertion-fee-credit.html
"If you originally listed the item during a promotion and your original Insertion Fee was lower than the Insertion Fee for your relisted item, you'll be credited for the
lower of the two fees if your item sells the second time. "
On eBay.ca, according to a post by a staffer a while ago, they do not expect to change the policy of crediting whatever the regular listing fees are if you relist an unsold listing from a promotional day.