Should he fulfil his promise to an idol-worshipper?

Praise be to Allah.

It is obligatory to fulfil any promises that do not go against the laws of Allaah. Allaah says (interpretation of the meaning):

“And fulfil (every) covenant. Verily, the covenant will be questioned about” [al-Israa’ 17:34]

Whether a promise is made to a Hindu or to anybody else it should be upheld so long as they do not go against the terms (under which they agreed to live under Muslim rule) or do any harm to Islam.

And Allaah is the Source of strength. May Allaah bless our Prophet Muhammad and his family and companions, and grant them peace.