Grouping in relational algebra with more than one grouping attribute
The usual notation is to write separately the grouping attributes from the aggregation functions, writing the attributes on the left of the γ symbol and the aggregation functions on the right, so your query should be something like this:

employeeId, date γ MAX(salePrice)→ largetSale(Sales)

This means:


make a group for each different combination of employeeId and date in the Sales relation, and
for each group produce a tuple with employeeId, date, maximum sale price of the group.