LOOKVALUE NEAREST : Nearest value based on other columns values

You could create a column in TableB like

Credit Limit =
VAR AvgPurchase = 'TableB'[Average purchase]
VAR Result =
    CALCULATE (
        MIN ( 'TableA'[Credit line amount] ),
        'TableA'[Credit line amount] >= AvgPurchase
    )
RETURN
    Result