Don't oppress orphans

And give unto orphans their property and do not exchange (your) bad things for (their) good ones; and devour not their substance (by adding it) to your substance. Surely, this is a great sin.

And if you fear that you shall not be able to deal justly with the orphan¬girls, then marry (other) women of your choice, two or three, or four but if you fear that you shall not be able to deal justly (with them), then only one or (slaves) that your right hands possess. That is nearer to prevent you from doing injustice.

And give to the women (whom you marry) their Mahr (obligatory bridal-money given by the husband to his wife at the time of marriage) with a good heart, but if they, of their own good pleasure, remit any part of it to you, take it, and enjoy it without fear of any harm (as Allâh has made it lawful).

An-Nisa 2-4

Comment
This verse refers to the case where an orphan girl is under the custody of a person and his custodian/guardian wants to marry her with the intention to eat up her property and doesn't pay her the mahr as much as she is being offered by other candidates or it is expected that she'll be given an offer more than that.