We try to integrate 3rd party POS through standard entities. We have an entity for most of what we need, but one thing is missing - the tender declaration transactions (=counting of money in till at the end of the day).

The entity RetailTransactionTenderDeclarationTrans does exist, but cannot be used for data import. It looks like it can only be used for updating existing transactions, as part of the Retail auditable transaction composite entity (https://learn.microsoft.com/en-us/dynamics365/commerce/edit-cash-trans).

It should be possible to import the whole dataset from a store using standard entities and there should be an entity for the tender declaration. I have heard that it might have been possible before, but was later deprecated.