gaw,
It makes sense that it wouldn’t be different than any other s’chirus. If you hire a secretary to work for you (and she does), could you exempt yourself from paying her because no kinyan was made in a cheifetz?
By s’chirus, you either have prior agreement, or the original Ratzon of the Socher to pay. Here, it seems that is not the case.
What this is similar to (IMHO) is a summer worker where there are “unpaid internships” as well (such as a Shaddchan that works for free). In that case, the requirement to pay should be made up front.