A.T. Robertson Commentary Luke 20:10

"And at the season he sent unto the husbandmen a servant, that they should give him of the fruit of the vineyard: but the husbandmen beat him, and sent him away empty." — Luke 20:10 (ASV)

At the season (καιρω). The definite season for the fruit like ο καιρος των καρπων (Matthew 21:34). That they should give (ινα δωσουσιν). Future indicative with ινα for purpose like the aorist subjunctive, though not so frequent.