The following is defined to work, and more complex versions of it seem to be a specifically supported workflow:

dnf install --assumeyes --store=mytransaction cowsay

# disconnect from all networks

dnf replay --assumeyes mytransaction

Up to and including 5.2.10.0 of dnf5 this worked as expected, with dnf replay using the RPMs downloaded into the mytransaction/packages/ folder that are also referenced within the mytransaction/transaction.json file itself.

However in 5.2.12.0 it does not work. Instead the dnf replay command ignores that the packages are already downloaded and available and attempts to download them again fresh. dnf replay is designed to be used on air-gapped/offline systems to install transactions that had their packages downloaded with them, so this is breakage of the command.