How to link data from two table with Formula requirement

Hi,

you can do this with both Power Query and Dax.

Power query:
Merge the Expense-table with the Travel-table on Employee ID. Create a column which is equal to 1 when Expense Date between Departure date and Return date, and filter this column on 1. Remove all columns except Employee ID, Expense Date, Nature and Travel ID

Dax:
New column the Expense-table:

TravelIdDax =
CALCULATE (
    SELECTEDVALUE ( Travel[Travel ID] );
    FILTER (
        Travel;
        Expanse[Employee ID] = Travel[Employee ID]
            && Expense[Expense Date] >= Travel[Departure Date]
            && Expense[Expense Date] <= Travel[Return Date]
    )
)

If you want to create a relationship between the tables on Travel ID, you should use power query, otherwise you will get a circular dependency error