All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
We have not come across any statement of the jurists regarding the situation which you asked about and whether or not such a condition is valid, but they stated a situation which may be of the same meaning.
The Hanbali book of Fiqh Ar-Rawdh al-Murbi’ Sharh Zaad al-Mustaqni’ reads, “If he (the husband) marries her (a woman) and says to her, ‘If I take another wife, I give you two thousand (as dowry), and if I do not take another wife, I give you one thousand,’ then the marriage is valid with the mentioned dowry, because the absence of a co-wife is one of the most coveted objectives of a woman...”
There is a narration by Imam Ahmad that forbids this situation, and it says that the mentioned dowry is invalid because the matter involves ignorance (of the compensation).
This ignorance is confirmed with regard to the wife serving her husband's parents. Therefore, it is more appropriate to avoid this condition between the spouses in order to avoid the difference of opinion of scholars.
Allah knows best.