Qala thalika baynee wabaynaka ayyama al-ajalayni qadaytu fala AAudwana AAalayya wallahu AAala (surah Qasas: 28)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(28:28) Moses replied, "Be it an agreement between me and you. Whichever of the two terms I complete, let there be no injustice to me after that; and Allah is a witness to what we have agreed upon. *39

Qaala zaalika bainee wa bainaka aiyamal ajalaini qadaitu falaa `udwaana `alaiya wallaahu `alaa ma naqoolu Wakeel

*39) Some people have taken this conversation between the Prophet Moses and the girl's father for a contract of marriage, and have started the dispute whether service under the father can be looked upon as a dower of the daughters marriage, and whether such external conditions can be laid down for the marriage contract; whereas the words of the verses under discussion themselves indicate this was not the contract of marriage but the initial proposal that is generally made before the execution of the marriage contract itself. After all, how can this be taken for a contract of marriage when it had not yet been decided which of the two girls was to be given away in marriage. The purport of the conversation was that the girl's father said, "I am prepared to marry one of my daughters to you provided that you promise that you will stay in my house for eight to ten years and help me in performing household chores, for I am old and have no son either, who could manage my properties. I have only daughters whom I have to send to perform outdoor duties. I want you to strengthen me as my would-be son-in-law. Lf you are willing to accept this responsibility, and do not intend to take away your wife soon after marriage, I will marry one of my daughters to you." The Prophet Moses himself was in search of a shelter at that time, so he accepted the proposal.
Evidently, it was a sort of contract that had been agreed upon between the two parties before the marriage. After this the actual marriage must have taken place according to the law and the dower also settled. There could be no question of including the condition of service in the marriage bond itself.